Morphology-controllable synthesis and enhanced photocatalytic activity of ZnS nanoparticles

Morphology-controllable synthesis and enhanced photocatalytic activity of ZnS nanoparticles

Accepted Manuscript Morphology–controllable synthesis and enhanced photocatalytic activity of ZnS nanoparticles Lixiong Yin, Dan Wang, Jianfeng Huang,...

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Accepted Manuscript Morphology–controllable synthesis and enhanced photocatalytic activity of ZnS nanoparticles Lixiong Yin, Dan Wang, Jianfeng Huang, Liyun Cao, Haibo Ouyang, Xiang Yong PII:

S0925-8388(15)31538-3

DOI:

10.1016/j.jallcom.2015.10.281

Reference:

JALCOM 35836

To appear in:

Journal of Alloys and Compounds

Received Date: 26 November 2014 Accepted Date: 29 October 2015

Please cite this article as: L. Yin, D. Wang, J. Huang, L. Cao, H. Ouyang, X. Yong, Morphology– controllable synthesis and enhanced photocatalytic activity of ZnS nanoparticles, Journal of Alloys and Compounds (2015), doi: 10.1016/j.jallcom.2015.10.281. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Morphology–controllable synthesis and enhanced photocatalytic activity of ZnS nanoparticles Yin Lixiong, Wang Dan, Huang Jianfeng*, Cao Liyun, Ouyang Haibo, Yong Xiang

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School of Materials Science and Engineering, Shaanxi University of Science and Technology, Xi’an, Shaanxi, 710021, China

Abstract: Well crystallized cubic sphalerite phase ZnS micro/nanocrystallites with controllable

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morphologies were successfully prepared by a facile and efficient microwave hydrothermal method. The syntheses were carried out by varying the Zn/S molar ratios. The phase compositions,

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morphologies and optical properties of the as–prepared samples were characterized by X–ray diffraction, scanning electron microscopy and UV–vis diffuse reflectance spectroscopy. Results show that with Zn/S molar ratio ranges from 2:1 to 1:2, the morphology of the prepared ZnS micro/nanocrystallites changed obviously. ZnS nanoparticles with good dispersibility prepared at Zn/S

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molar ratio of 1:2, which average size is about 20 nm, shows high photocatalytic activity to degrade Methyl orange (MO). The degradation efficiency of ZnS nanoparticles reaches 97.4% under UV irradiation for 20 min. The good ultraviolet absorbing ability and big specific surface area of ZnS

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nanoparticles are believed to have a positive impact on improving the final degradation rate and degradation efficiency of MO in our research.

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Keywords: ZnS; Nanostructured materials; Photocatalytic activity; Chemical synthesis Introduction

Environmental pollution coming from organic pollutants and toxic waste water has drawn more

and more attention in recent years [1–3]. Now increasing interest of heterogeneous photocatalysis for

*

Corresponding author: School of Materials Science and Engineering, Shaanxi University of Science and Technology, Xi’an

710021, China. E-mail addresses: [email protected] (Huang Jianfeng). Tel/fax: +86 29 86168802.

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the treatment of recalcitrant and toxic pollutants present in the wastewater has been aroused [4, 5]. Because of high efficiency, commercial availability and high chemical stability, the semiconductor

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photocatalysis is considered as one of the most promising environmental pollution remediation processes [6]. The process of heterogeneous photocatalysis is a kind of advanced oxidative process. It is based on electron–hole pairs created in semiconductor materials by the absorption of photons, which

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can further generate free radicals such as hydroxyl in the system to redox compounds absorbed on the surface of photocatalysts [7, 8].

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ZnS as an important II–VI semiconductor material with the band gap energy (Eg) of 3.6 eV, has been studied extensively as active photocatalysts owing to its high energy conversion efficiency and the relatively negative redox potential of its conduction band [9, 10]. To enhance the photocatalytic activity of ZnS, extensive researches have been performed by controlling the morphology [11],

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crystallinity [12], and surface area [13]. In addition, considered the size effects originate primarily from the size quantization in nanoscale that can change the band gap of semiconductor and size–related surface characteristics such as surface area and defects, ZnS nanoparticles with big

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specific area have gained continues attentions [14]. The development of facile, cost–effective template–free methods suitable for the large–scale synthesis of ZnS nanoparticles with good

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dispersibility is of great importance and challenge for their industrial applications [13]. Many recent literatures have reported that hydro/solvothermal method [6, 15], chemical vapor

deposition [16], precipitation method [17] and thermal decomposition [18] were widely used to prepare ZnS. It has been demonstrated that hydrothermal method is the most effective in fabricating well crystallized ZnS [6]. However, it takes long time to synthesize ZnS by traditional hydrothermal method, so in this paper we propose a facile and efficient microwave hydrothermal method to prepare

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morphology–controlled ZnS micro/nanocrystallites by just adjusting the Zn/S molar ratios in the reaction system. The prepared ZnS micro/nanocrystallites with multistage micro/nanospheres and

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nanoparticles morphologies exhibit different optical and photocatalytic properties. The influence of different Zn/S molar ratios on the structure and photocatalytic activity of ZnS was particularly investigated. Different kinds of dyes: anionic (Methyl orange), cationic (Rhdamine B) and neutral

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(Neutral red) dyes were used to test the photocatalytic response property of the prepared ZnS nanoparticles. Moreover, the photocatalysis degradation reaction kinetics of MO, which catalyzed by

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the as–prepared ZnS micro/nanocrystallites was also researched in this paper. Experiment

Analytical Zn(NO3)2·6H2O, thioacetamide (TAA), NH3·H2O and HNO3 were used as raw materials without any purification. First, 25 mmol Zn(NO3)2·6H2O was dissolved into 50 mL H2O,

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then 12.5 mmol, 25 mmol and 50 mmol TAA were added with magnetic stirring to form the precursor solutions, respectively. The pH value of the reaction solutions were adjusted by NH3·H2O (Wt%=10%) and HNO3 (0.5 mol·L-1) to be 7.00. The precursor solutions were transferred to 100 mL Teflon–lined

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stainless steel autoclaves with the filling capacity of 50% and maintained in an MDS–10 microwave hydrothermal system at 170 °C with the operating power of 400 W for 2 h. After the system terminated

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and cooled down to room temperature, the precipitants were centrifuged and washed by distilled water and anhydrous ethanol for several times. Afterwards, these precipitants were finally dried in vacuum drying oven at 60 °C for 3 h. The crystalline microstructure of the as–prepared powder was characterized by a powder X–ray diffraction (XRD, Rigaku D/max–2000) with Cu Kα radiation (λ=0.15406 nm) at 40 kV and 40 mA in the 2θ range of 10°~60°. The morphology of the sample was observed by field emission scanning

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electron microscopy (FE–SEM, Hitachi S–4800, Acceleration voltage: 3 kV). UV–vis diffuse reflectance spectrum of the sample was measured by Shimadzu UV–2450 UV–vis spectrophotometer.

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Photocatalytic activities of the prepared ZnS micro/nanocrystallites were evaluated by photocatalytic degradation of 10 mg·L–1 Methyl orange (MO), Rhdamine B (RhB) and Neutral red (NR) aqueous solution. The photocatalytic activity tests were carried out by employing a BL–GHX–V

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photocatalytic reactor (Xi’an, BILOBN, Co. Ltd.) with a 300 W mercury lamp as UV light source. The loading amount of catalysts was 0.5 g·L–1. Before illumination, the suspensions of dyes with catalysts

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were magnetically stirred in the dark for 30 min, after dispersing in an ultrasonic bath for 5 min, to ensure the establishment of an adsorption–desorption equilibrium between catalysts and dyes. Then, the solution was exposed to a 300 W mercury lamp under magnetic stirring. By the irradiation time prolong, 6 ml of the solution was collected by centrifugation each 5 min. The concentrations of the

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remnant dyes in the collected solution were monitored by UV–vis spectroscopy (Unico UV–2600). In the process of photocatalytic reaction, the degradation efficiency of dyes was calculated by Eq. (1): Degradation efficiency % = 1 − C ⁄C  ∗ 100%

(1)

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Where C0 represents the initial concentration of dye aqueous solution and Ct represents the concentration of dye aqueous solution after different minutes of UV irradiation. Results and discuss

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3.

3.1. Phase analysis

(Fig.1. is supposed to be here.) Fig.1 shows the XRD patterns of the samples prepared at different Zn/S molar ratios range from

2:1 to 1:2. All the diffraction peaks of the as–prepared samples shown in Fig.1 can be readily indexed to the pure typical cubic sphalerite ZnS (JCPDS No.05–0566). Main reflection peaks of ZnS become

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sharper with the Zn/S molar ratio varies from 2:1 to 1:2, which indicates that excessive S2- is good to improve the crystalllinity of ZnS.

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3.2. Morphological analysis (Fig.2. is supposed to be here.)

SEM images of the as–prepared ZnS micro/nanocrystallites are shown in Fig.2. It can be

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obviously observed that the morphologies of ZnS prepared at Zn/S molar ratio varies from 2:1 to 1:1 are spheres with different sizes and surface morphologies. ZnS nanoparticles with uniform size and

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good dispersibility were achieved at Zn/S molar ratio of 1:2. With the increase of TAA, the concentration of S2- increased, the size of the samples gets smaller. This suggests TAA play an important role in the morphology control of ZnS micro/nanocrystallites. (Fig.3. is supposed to be here.)

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TAA can release mass S2- in short time, which can easily react with Zn2+ to form ZnS crystal nucleus. Fig.3 displays the schematic diagram of the possible growth pattern of ZnS micro/nanospheres and nanoparticles prepared at different Zn/S molar ratios. When S2- is insufficient

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(Zn/S molar ratio equals 2:1), ZnS microcrystallites with microspheres morphology, which average diameter is about 1.2 µm were prepared. The surfaces of the microspheres were covered by small

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nanorods. In this case, ZnS microsphere may be assembled by small nanorods in the growth and crystallization process. ZnS nanocrystallites prepared at Zn/S molar ratio equals 1:1 with the morphology of multistage nanospheres, which average diameter is about 500 nm, were assembled by a lot of irregular nanoparticles. This means, when Zn/S molar ratio is according with the stoichiometric ratio to form ZnS, the ZnS crystal nucleuses competitively grow to be nanoparticles and random aggregation to assemble form nanospheres to decrease the higher surface energy of the nanoparticles.

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When S2- is excessive (Zn/S molar ratio equals 1:2), mass ZnS crystal nucleuses formed to grow and crystallize of ZnS nanoparticles, which average size is only about 20 nm.

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3.3. Optical and photocatalytical properties (Fig.4. is supposed to be here.)

The UV–visible diffuse reflection spectra of the as–prepared ZnS micro/nanocrystallites are

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shown in Fig.4. It can be seen that there exists a strong absorption edge below 350 nm for all samples. The steep shape of the spectra indicates that the samples have good absorption property of ultraviolet.

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The absorption band edge of the samples prepared at Zn/S molar ratio of 1:2 is red–shift with the enhanced optical absorbing ability, which may be attributed to their smaller sizes. (Fig.5. is supposed to be here.)

Methyl orange (MO), Rhdamine B (RhB) and Neutral red (NR) were used as organic pollutants

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to evaluate the photocatalytic activity of the as–prepared ZnS nanoparticles, corresponding photocatalytic results are shown in Fig.5 (a). The photocatalytic results exhibit that ZnS nanoparticles can mainly degrade MO and NR in only 20 min, which degradation efficiency reaches 97.4% and

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88.8%. However, the degradation efficiency is only 70.4%, when RhB was used as the organic pollutant. These results indicate that the as–prepared ZnS nanoparticles have good responsiveness to

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the anion (MO) and neutral (NR) dyes, especially the anion dye. Therefore, the photocatalytic degradation of MO was used to evaluate the photocatalytic activity of the as–prepared ZnS micro/nanocrystallites.

The photocatalytic results of MO degraded by ZnS micro/nanocrystallites prepared at different Zn/S molar ratios are shown in Fig.5 (b). The adsorption test shows that the adsorption–desorption equilibrium between ZnS nanoparticles and MO was achieved after the dark stirring for 30 min. The

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blank test demonstrates that the degradation of MO was very slow without photocatalysts. When the prepared ZnS micro/nanocrystallites were used as photocatalysts, the degradation of MO was obvious

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as irradiation time prolongs. The photocatalytic results exhibit that the ZnS nanoparticles prepared at Zn/S molar ratio of 1:2 can mainly degrade MO in only 20 min with the degradation efficiency reaches 97.4%. Whereas, the degradation efficiency of the sample prepared at Zn/S molar ratio of 2:1 and 1:1

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only reaches 56.6% and 64.5%. In addition, comparing with the well known commercial photocatalyst P25, which degradation efficiency of MO is only 75.0%, the photocatalytic activity of the prepared

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ZnS nanoparticles is promising. The photocatalytic reactions are typically surface–based processes, the photocatalytic efficiency is closely related to the specific surface area, morphology and microstructure of the materials [19]. Therefore, the better photocatalytic activity of ZnS nanoparticles may be due to the smaller size and good ultraviolet absorbing property.

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The degradation rate is another way to directly evaluate the photocatalytic activity of the as–prepared samples. The heterogeneous photocatalytic reaction between ZnS micro/nanocrystallites and MO aqueous solution can be described by the pseudo–first–order kinetics, which is rationalized in

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terms of the Langmuir–Hinshelwood model (L–H) modified to accommodate reactions occurring at a solid–liquid interface [20, 21]. The photocatalytic activity of ZnS micro/nanocrystallites for

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degradation of MO obeys the pseudo–first–order reaction kinetics and its expression is as follows:

dc / dt = − KcC

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ln(C 0 / Ct ) = Kct

(3)

Where Kc is the rate constant of the pseudo–first–order reaction. A plot of ln(C0/Ct) versus the UV irradiation time for the MO photodegradation, which catalyzed by ZnS micro/nanocrystallites and P25, is shown in Fig.6. A near linear relation between ln(C0/Ct) and the irradiation time have been observed,

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which explains the photodegradation of MO by ZnS micro/nanocrystallites and P25 follows the pseudo–first–order kinetics. The calculated Kc, the corresponding first–order kinetic equation and the

(Fig.6. is supposed to be here.)

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R2 values are summarized in Table1.

Table 1 Parameter and linear kinetic equation of photocatalytic reaction of the samples prepared at

Kc

First–order kinetic equation

R2

2:1

0.04172

ln(C0 /Ct)=0.04172t –0.00521

0.99231

1:1

0.04750

ln(C0 /Ct)=0.04750t + 0.1093

0.94136

1:2

0.16479

ln(C0/Ct)=0.16479t + 0.44761

0.91618

P25

0.06560

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different Zn/S molar ratios and P25

ln(C0 /Ct)=0.06560t + 0.0913

0.98777

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The high value of R2 (>0.90) demonstrates the pseudo–first–order kinetic equation fit the

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photocatalytic degradation of MO perfectly. The higher the pseudo–first–order rate constant (Kc) suggests the more outstanding photocatalytic activity. The fast degradation rate to degrade MO aqueous solution was obtained by ZnS nanoparticles prepared at Zn/S molar ratio of 1:2, which is

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almost quadruple that prepared at Zn/S molar ratio of 2:1. The fast degradation rate may be due to the good ultraviolet absorbing ability and smaller size of the as–prepared ZnS nanoparticles. The

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nanoscale facilitates the transfer of photogenerated electron–hole pairs to the surface, which can inhibit electron–hole recombination and enhance the photocatalytic activity [13]. The good ultraviolet absorbing ability and big specific surface area of the ZnS nanoparticles are believed to greatly affect the final degradation rate and degradation efficiency of MO in our research. In addition, the degradation rate of ZnS nanoparticles is about two point five times to P25, which means the photocatalytic activity of the prepared ZnS nanoparticles is promising.

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4.

Conclusion In summary, ZnS micro/nanocrystallites with controllable morphologies have been successfully

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prepared by a facile and efficient microwave hydrothermal process. It is found that different Zn/S molar ratios significantly influence the structure and photocatalytic activity of the prepared ZnS micro/nanocrystallites. With Zn/S molar ratio ranges from 2:1 to 1:2, the morphology of the prepared

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samples changed obviously. Promising photocatalytic activity of the ZnS nanoparticles prepared at Zn/S molar ratio of 1:2 with the degradation rate reaches 97.4% under UV irradiation for 20 min,

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which may originate from the small size and good ultraviolet absorbing ability of the sample. Comparing with the well known photocatalyst P25, ZnS nanoparticles shows promising photocatalytic activity under ultraviolet irradiation. Furthermore, try to broaden the response of the as–prepared ZnS nanoparticles to visible–light is our main research objective in the future.

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Acknowledgments

The authors are grateful to National Key Technology R&D Program (No. 2013BAF09B02), National Natural Science Foundation of China (No. 51472152), Shaanxi Provincial Industrial Science

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and technology key project (No. 2014K08–39), Science and Technology Planning Project of Xianyang City Shaanxi Province (No. 2013JK02–11), Research Startup Funds for Doctors in Shaanxi University

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of Science and Technology (No. BJ12–12) and Graduate Innovation Fund of Shaanxi University of Science and Technology. References

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Figure captions

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624(2014)233–238.

Fig.1. XRD patterns of the samples prepared at different Zn/S molar ratios

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Fig.2. SEM images of the as–prepared ZnS micro/nanocrystallites prepared at different Zn/S molar ratios (a) 2:1 (b) 1:1 (c) 1:2

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Fig.3 Schematic diagram of the possible growth pattern of ZnS micro/nanospheres and nanoparticles prepared at different Zn/S molar ratios

Fig.4. UV–vis diffuse reflectance spectrum of the as–prepared ZnS micro/nanocrystallites prepared at different Zn/S molar ratios (a) 2:1 (b) 1:1 (c) 1:2 Fig.5. (a) Photocatalytic results of MO, RhB and NR degraded by ZnS nanoparticles (b) Photocatalytic results of MO degraded by different kinds of photocatalysts

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Fig.6. Relationship between ln(C0/Ct) and UV irradiation time of the ZnS micro/nanocrystallites prepared at different Zn/S molar ratios (a) 2:1 (b) 1:1 (c) 1:2 and (d) relationship between

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ln(C0/Ct) and UV irradiation time of P25

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Table 1 Parameter and linear kinetic equation of photocatalytic reaction of the samples prepared at different Zn/S molar ratios and P25

R2

First–order kinetic equation

2:1

0.04172

ln(C0 /Ct)=0.04172t –0.00521

1:1

0.04750

ln(C0 /Ct)=0.04750t + 0.1093

0.94136

1:2

0.16479

ln(C0/Ct)=0.16479t + 0.44761

0.91618

P25

0.06560

ln(C0 /Ct)=0.06560t + 0.0913

0.98777

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Kc

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0.99231

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Sample

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Morphology–controlled ZnS were achieved by adjusting the Zn/S molar ratios. Size–related photocatalytic activity of ZnS micro/nanocrystallites was shown. Different kinds of dyes were used to test the photocatalytic activity of ZnS nanoparticles.

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The photocatalysis degradation reaction kinetics of MO was researched.