Facile synthesis of ZnS nanoparticles and their excellent photocatalytic performance

Facile synthesis of ZnS nanoparticles and their excellent photocatalytic performance

Author's Accepted Manuscript Facile synthesis of ZnS nanparticles and their excellent photocatalytic performance Fengjuan Chen, Yali Cao, Dianzeng Ji...

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Author's Accepted Manuscript

Facile synthesis of ZnS nanparticles and their excellent photocatalytic performance Fengjuan Chen, Yali Cao, Dianzeng Jia

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S0272-8842(15)00158-3 http://dx.doi.org/10.1016/j.ceramint.2015.01.111 CERI9875

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11 January 2015 21 January 2015 22 January 2015

Cite this article as: Fengjuan Chen, Yali Cao, Dianzeng Jia, Facile synthesis of ZnS nanparticles and their excellent photocatalytic performance, Ceramics International, http: //dx.doi.org/10.1016/j.ceramint.2015.01.111 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 galley proof before it is published in its final citable 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.

Facile synthesis of ZnS nanparticles and their excellent photocatalytic performance Fengjuan Chena,b, Yali Caoa, Dianzeng Jiaa*

a

Key Laboratory of Advanced Functional Materials of Autonomous Region, Key

Laboratory of Clean Energy Material and Technology of Ministry of Education, Institute of Applied Chemistry, Xinjiang University, 830046, Xinjiang, P. R. China b

School of Physics Science and Technology, Xinjiang University, Urumqi Xinjiang,

830046, Xinjiang, P. R. China

Corresponding author. Tel.: +86 0991 8583083; Fax: +86 0991 8588883. E-mail address: [email protected] Abstract: In this work, ZnS nanoparticles were successfully fabricated by a simple low-temperature solid-state method. The phase composition, morphology, pore structure and optical property of the samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), nitrogen absorption-desorption specific surface area (BET) and UV-vis spectroscopy (UV). The photocatalytic activities of the as-synthesized samples were evaluated by the color removal of methyl orange (MO) under UV irradiation. The effects of surfactants and reactants on the microstructures and 1

photocatalytic activity of the samples were investigated. The results revealed that the surfactant played an important role in the microstructures and photocatalytic activity. It was found that the as-synthesized ZnS sample assisted by sodium dodecyl sulfonate (SDS) exhibited the highest photocatalytic activity with nearly 95 % of MO decomposed after irradiation for 60 min. The excellent photocatalytic activity of the sample can be ascribed to the large specific surface area and unique morphology. Keywords: ZnS; Photocatalysis; Low-temperature solid-state method

1. Introduction

Semiconductor photocatalysis technology has been widely used to degrade pollutants and decompose water for producing hydrogen, due to the advantages of low energy-input, easy operation, clean and efficient [1-3]. To date, a number of oxides and sulfides semiconductor nanomaterials have been prepared and applied to the above photocatalytic application [4-6]. In particular, zinc sulfide (ZnS) nanomaterial has attracted wide attention, because of unique band structure and high capability to decompose the organic pollutants. For example, Yu et al. had prepared monodisperse ZnS hollowball by hydrothermal method, which exhibited a high photocatalytic activity on Rhodamine B [7]. Yu et al. found that the as-synthesized ZnS microspheres exhibit excellent photocatalytic performance on decomposing water to produce hydrogen, which is attributable to the porous structure and large specific surface area [8].

2

Obviously, structure of photocatalyst is the main factor to affect the performance, and ZnS photocatalyst with different structures will have potential applications in energy and environment areas. The above statements encourage us to get ZnS with excellent photocatalytic performance by controlling the material structure. For example, people have synthesized ZnS nanoribbons, ZnS hollow particle and ZnS nanospheres by different methods [9-11], and studied the effect of the structure on the photocatalytic property. Such as Liu et al. fabricated layered structure of ZnS nanorods by a wet chemical method, which exhibited superior photocatalytic activity in the color removal of azo dyes, that is because the layered structure increases the contact area between photocatalyst and pollutant [12]. Zhang et al. synthesized Bi3+ doped ZnS hollow nanospheres by ion exchange method, which exhibited excellent photocatalytic activity on decomposing water to produce hydrogen, attributing to the changes in the microstructure by the doping, resulting in a transition level, and helping for the effective separation of photogenerated carriers [13]. Lu et al. obtained ZnS hierarchical structure by thermal evaporation method, which showed well photocatalytic activity on the color removal of methylene blue [14]. Hence, the structure of photocatalyst plays a key role in the performance. However, those methods usually come down to rigor requirements, such as high reaction temperature, complicated equipments, high power consumption and so forth [15-19]. Thus, a simple, economical and effective way to obtain ZnS nanomaterials with unique structure and excellent photocatalytic performance is still a

3

big challenge. In this study, ZnS nanoparticles have been successfully synthesized via a simple low-temperature solid-state chemical method, which has the merits of simplicity, low

reaction temperature, short time, low cost and high yield. In order to prevent the aggregation of ZnS nanoparticles during reaction process, several surfactants were added to the reaction system. The as-synthesized samples were used as photocatalysts for the color removal of methyl orange (MO) under UV irradiation. The effects of surfactants and reactants on the microstructures and photocatalytic activity of the samples were discussed. The results indicated that the surfactant played key role in the microstructures and photocatalytic activities of the samples.

2. Experimental section

2.1. Materials and synthesis

All reagents used in this study were of analytical grade (sinopharm) and used without further purification. In order to investigate the effects of surfactant types on the microstructures and properties of samples, polyethylene glycol 400 (PEG-400, nonionic surfactant), cetyltrimethyl ammonium bromide (CTAB, cationic surfactants) and sodium dodecyl sulfonate (SDS, anionic surfactants) were selected as surfactants. (a) In a typical synthesis, 5 mmol zinc acetate and 5 mmol thioacetamide were

4

weighed respectively and ground in an agate mortar for about 10 min, then mixed and ground for 60 min to assure entire reaction. The reaction started as soon as the reactants came into contact, accompanied by the release of heat and evaporation of water vapor. In the end, the mixture was washed with distilled water for several times to remove by-products. The product was dried in air at 60 oC for 6 h, and the obtained sample was named as ZS1. (b) In a typical synthesis, 5 mmol zinc acetate was accurately weighed and ground for about 10 min, and then 3 mL PEG 400 and 5 mmol thioacetamide powder were added. The mixture was ground for 60 min to ensure a full reaction. Then the mixture was washed with distilled water and alcohol for several times to remove impurity ions and PEG 400. The product was dried in air at 60 oC for 6 h, and the resulting sample was designated as ZS2. To investigate the effects of surfactants on the microstructures of samples, another experiment was adopted following the same procedure as mentioned above, except that PEG 400 was replaced by CTAB and SDS, and the obtained samples were designated as ZS3 and ZS4, respectively. (c) To investigate the effect of reactants on the microstructures of samples, another experiment was adopted following the same procedure as mentioned above, except that thioacetamide was replaced by sodium thiosulfate as sulfur source, zinc acetate was replaced by zinc chloride as zinc source, and the obtained samples were designated as ZS5 and ZS6, respectively.

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2.2. Characterization

The phase structures of the samples was examined via X-ray diffraction (XRD) using a Bruker D8 Advance diffractometer equipped with Cu Ka radiation (k = 1.5418 Å) under the operation conditions of 40 kV and 40 mA at scanning rate of 0.04o s-1 in the range of 10-80o. Morphologies of the samples were analyzed with transmission electron microscopy (TEM, JEOS, H-600, 100 kV). In the preparation of samples for TEM observation, the materials were first suspended in ethanol and sonicated over 10 min. Subsequently, a drop of the supernatant dispersion was dropped onto a cupper grid, which was dried in air at room temperature and kept in vacuum for 20 min before TEM observation. Specific surface area and porosity measurements were carried out on a Micromeritics ASAP 2050 instrument at liquid-nitrogen temperature using nitrogen gas as the adsorbate, in which all samples were previously degassed at 200 oC for 6 h in flowing N2. The specific surface area was calculated using the Brunauer-Emmett-Teller (BET) method. The pore size distribution was determined using the density functional theory (DFT) model based on nitrogen desorption isotherm. UV-vis absorption spectra of the samples were conducted on a Hitachi U-3010 spectrophotometer in the range of 300-700 nm. The photocatalytic experiments were conducted in an XPA-1 photochemical reactor (Xujiang Electromechanical Plant, Nanjing, China).

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2.3. Photocatalytic activity test

Photocatalytic activities of the samples were measured by the color removal of methyl orange (MO) under UV irradiation using a 300 W mercury lamp (the light intensity is 12.2 mW). Typically, 50 mg of photocatalysts were added into 100 mL of 10 mg/L MO aqueous solution. The suspension was continuously stirred for 60 min in the dark to ensure the adsorption-desorption equilibrium between the photocatalyst and the MO. The solution was then shined under UV irradiation. At a given irradiation time, 5 mL of the suspension was collected and centrifuged to remove the photocatalyst, then analyzed by recording the UV-vis spectra of MO at the maximum absorption wavelength. All the experiments were conducted at room temperature. 3. Results and discussion

3.1. Effect of surfactants on the microstructure and photocatalytic activity of the

products

3.1.1. Phase sturctures and morphologies Fig. 1 is here. PEG-400, CTAB and SDS were selected as surfactants to investigate the effect on the phase structures and morphologies of the samples,. Fig. 1 shows the XRD patterns of the samples prepared at different surfactants. It indicates that all diffraction peaks can be indexed to standard pattern of cubic phase ZnS structure (JCPDS no. 05-0566) [20]. No 7

other impurities were detected in the samples, suggesting that pure ZnS were obtained. Moreover, the XRD peaks intensities of samples prepared with surfactants

are

enhanced, and the widths become narrower than that of the sample without surfactant, implying the formation of stronger crystallites in the samples synthesized with surfactants [21]. Fig. 2 is here. The morphologies of samples prepared at different surfactants were examined by TEM, and the results are presented in Fig. 2. It can be found that different aggregation states of ZnS samples were obtained when different surfactants were added. Fig. 2a revealed that the sample Z1 (without surfactant) was mainly composed of ZnS nanoparticles with an average diameter about 30 nm, and occurred some agglomeration to a certain degree. When adding PEG-400 and CTAB to the reaction, more uniform and dispersed ZnS nanoparticles were obtained. When the surfactant SDS was added, the resulting ZnS nanospheres were assembled by ZnS nanocrystals and stacked loosely. It indicates that the above sample will have better photocatalytic performance, which is consistent with the highest photocatalytic activity in the later. As can be seen from the statement, the introduction of surfactants change the aggregation state of the basic structural unit of the samples, therefore they act as a dispersing agent in the system. The difference in the aggregation degree is mainly due to the difference in the molecular structures of different surfactants.

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3.1.2. Specific surface area and porosity

Fig. 3 is here. Since the specific surface area has an important effect on the photocatalytic activity of the samples, nitrogen adsorption-desorption measurements were conducted [22,23]. Fig. 3 shows nitrogen adsorption-desorption isotherms and the corresponding density functional theory (DFT) pore size distributions (inset) of the samples prepared at different surfactants. According to the classification of International Union of Pure and Applied Chemistry (IUPAC) [24], all the samples exhibited a

Type IV

adsorption-desorption isotherms curves, indicating the presence of mesopores stemmed from the interparticles [25], which can also be confirmed by the corresponding pore size distribution curves (inset of Fig. 3). The isotherms curves of the samples can be ascribed to Type H2, indicating that the shapes of the holes are "ink bottle" and clearance holes between close packing of spherical particles. The determined specific surface areas (SBET) of the samples were listed in Table 1. It can be found that the specific surface areas increased with the addition of surfactants. As noted in above, the dispersion of the surfactants changes the aggregation state of the samples, and then changes the pore structure, leading to the increase of specific surface area.

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3.1.3. UV-vis analysis

Fig. 4 is here. Since the photoabsorption property of semiconductors plays a crucial role in the photocatalytic activity, the optical absorption properties of the samples were investigated by UV-vis absorption spectra [26]. Fig. 4 exhibits the UV-vis spectra of the samples prepared with different surfactants. It can be seen that all the samples have strong absorptions in the ultraviolet region, and steep absorption edges can also be found. Comparing with sample ZS1 (without surfactant), the absorption spectra of the samples prepared with the surfactants are significantly enhanced in the ultraviolet region, but the difference of their absorption edges is not very obvious. In addition, in comparison with sample ZS1, absorption edges of sample ZS3 and sample ZS4 both have a slight blue shift, while sample ZS2 has an obvious red shift, indicating that surfactants can adjust the band gap and electronic structure of the samples.

3.1.4. Photocatalytic activity

Fig. 5 is here. The photocatalytic activities of the as-synthesized ZnS samples prepared with different surfactants were evaluated by the color removal of methyl orange (MO) under UV irradiation, and the results were presented in Fig. 5. Fig. 5a shows the changes of MO concentration (C/C0) as a function of irradiation time, where, C0 and C are the dye

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concentration at time 0 and t min, respectively. It can be noticed that without catalyst the MO dye showed only a slight color removal, indicating that the photolysis of MO can be neglected and the photocatalyst plays a key role in the color removal of MO. From the experimental results, it can be seen that the photocatalytic activities of the samples were different with the addition of surfactants. Wherein, sample ZS4 with the assistance of SDS exhibited the highest photocatalytic activity, and nearly 95 % of MO was decolorized after exposure for 60 min. As for sample ZS1 (without surfactant), the color removal rate was 47 % within the same time. Thus, it is evident that sample ZS4 exhibited higher photocatalytic activity than that of sample ZS1. Comparing with the previously reported degradation of the same dye (MO) in the presence of ZnS, it can be found that the as-synthesized ZnS nanoparticles with the assistance of SDS show relatively higher photocatalytic activity [27]. So, the as-synthesized ZnS nanoparticles can act as a more efficient photocatalyst. Fig. 5b presents the absorption spectra of MO aqueous solution in the presence of sample ZS4 under UV irradiation at different time intervals. As the exposure time prolonging, the intensities of absorption peaks rapidly decreased due to the color removal of MO. About 95 % of MO was degraded within 60 min by the sample ZS4, suggesting the nearly complete color removal of MO. The result showed that the sample ZS4 possessed excellent photocatalytic activity in the color removal of MO. The enhanced photocatalytic activity of sample ZS4 can be ascribed to many factors, such as specific surface area and nanostructure. It is generally accepted that the

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photocatalytic process is related to the adsorption and desorption of molecules on the surface of catalysts [28]. Thus, large specific surface area is beneficial to absorb more light and increase more unsaturated surface coordination sites to improve the photocatalytic performance. In addition, large specific surface area can also offer more opportunities for the diffusion and mass transportation of MO molecules in the photochemical color removal reaction [29]. As shown in Table 1, the determined specific surface area of sample ZS4 was 190.4 m2/g, higher than that of others, implying that sample ZS4 possessed higher photocatalytic activity. On the other hand, ZnS nanostructure is critical for photo-generated electron-hole capture and interface transfer. As shown in Fig. 2, sample ZS4 are composed of loose nanospheres assembled by ZnS nanocrystals, which facilitates the rapid photo-generated electron-hole transport, thus improving the photocatalytic activity. Finally, the adjustment of electronic structure and band gap by surfactants can also affect the photocatalytic activity of the samples. As shown in Fig. 4, comparing with sample ZS1, the blue shift of absorption edge for sample ZS4 is most obvious, indicating a larger band gap and larger redox ability, implying the higher photocatalytic activity. In addition, as a anionic surfactants, SDS has a strong interaction with zinc ion, and then there are organic groups attaching on the surface of the sample. The hydrophilic organic groups at the surface can afford excellent water solubility for the sample, which is favorable for adsorbing more pollutants, then leading to high photocatalytic efficiency.

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3.2. Effect of reactants on the microstructure and photocatalytic activity of the

products

3.2.1. Phase structures and morphologies Fig. 6 is here. To study the effect of reactants on the phase structures and morphologies of the samples, the ZnS samples were prepared at sodium thiosulfate as the sulfur source and at zinc chloride as the zinc source. Fig. 6 shows the XRD patterns of the samples prepared at different reactants. As can be seen from the patterns, changing sulfur source and zinc source, the diffraction peaks of the samples were consistent with the standard pattern of cubic phase ZnS structure (JCPDS no. 05-0566). No other impurity peak was observed, indicating that the change of sulfur source and zinc source has no effect on the phase structure of ZnS. In addition, the diffraction peaks of sample ZS5 and sample ZS6 became broaden, suggesting small sizes of the samples, which were also consistent with the TEM results in Fig. 7. Fig. 7 is here. Fig. 7 exhibits the TEM images of the samples prepared at different reactants. It can be found that different aggregation states of ZnS samples were obtained after changing reactants. Such as using sodium thiosulfate as the sulfur source, the sample ZS5 was composed of ZnS nanoparticles with diameter about 50 nm (Fig. 7a), which was 13

different with the sample ZS4, probably due to the slow release rate of S2- coming from sodium thiosulfate, leading to the crgystal nucleus of new ZnS crystal nuclei not enough. When using zinc chloride as zinc source, the as-synthesized ZnS nanoparticles were about 100 nm in diameter, and piled together loosely. As for zinc chloride, the pH was small, and then accelerated the decomposition of thioacetamide. In addition, since the volume of Cl- was smaller than that of CH3COO-, reaction resistance was smaller and growth rate of nucleation was faster, therefore the size of ZnS nanoparticles were larger. Hence, changing sulfur source and zinc source can adjust aggregation states of the samples.

3.2.2. BET specific surface area and porosity

Fig. 8 is here. Fig. 8 shows nitrogen adsorption-desorption isotherms and the corresponding density functional theory (DFT) pore size distributions (inset) of the samples prepared at different reactants. According to IUPAC classification, the samples exhibited Type IV adsorption-desorption isotherms curves, indicating the existence of mesoporous structure [30], which can also be confirmed by the corresponding pore size distribution curves (inset of Fig. 8). The pore size of sample ZS5 was about 70 nm, while that of sample ZS6 was about 20 nm. The isotherms curves of the above samples can be ascribed to Type H3 [31], which was typical of the open slit-shaped capillaries with wide bodies and narrow necks [32]. Furthermore, comparing with sample ZS5, the pore

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size of sample ZS6 was smaller and the distribution was narrower, therefore the specific surface area was larger, which was in agreement with the results in Table 2.

3.2.3. UV-vis analysis

Fig. 9 is here. Fig. 9 exhibits the UV-vis spectra of the samples prepared at different reactants. It can be seen that all the samples have strong absorptions in the ultraviolet region, and steep absorption edges can also be noted. Comparing with sample ZS1 (without surfactant), the absorption spectra of sample ZS5 and sample ZS6 are significantly enhanced in the ultraviolet region. In addition, in comparison with sample ZS1, absorption edges of sample ZS5 and sample ZS6 have obvious blue shifts due to quantum size effect [33], suggesting smaller sizes of the samples, which can be confirmed by the TEM results in Fig. 7.

3.2.4. Photocatalytic activity

Fig. 10 is here. The effect of reactants on the photocatalytic activity of the as-synthesized ZnS samples was studied. Fig. 10 shows the changes of MO concentration (C/C0) as a function of irradiation time. As can be seen from the Fig. 10, comparing with sample ZS1, the photocatalytic activities of sample ZS5 and sample ZS6 improved. Wherein, sample ZS5 by changing sulfur source exhibited higher photocatalytic activity than that 15

of sample ZS6, but still slower than that of sample ZS4, attributing to the high specific surface area and loose structure. These results suggest that sulfur source and zinc source have little effect on the photocatalytic properties of samples, while the surfactant played more.

4. Conclusions

In summary, a simple low-temperature solid-state method was employed for the synthesis of ZnS nanoparticles, which has the virtues of simplicity, low cost, high yield and low environmental impact. XRD, TEM, BET and UV were used to characterize the phase composition, morphology, pore structure and optical property of the samples. The results revealed that the surfactant played a crucial effect on the microstructure and photocatalytic activity of the samples. The as-synthesized ZnS sample assisted by SDS exhibited the highest photocatalytic activity in comparison with others, with nearly 95 % of MO decomposed after irradiation for 60 min, due to the large specific surface area and unique morphology.

Acknowledgments

This work was financially supported by Scientific Research Program of the Higher Education Institution of Xinjiang (No. XJEDU2014S004), the Natural Science Foundation of Xinjiang Province (No. 2014211A013), the National Natural Science Foundation of China (Nos. 21271151 and 21361024). 16

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Figure Captions Fig. 1. XRD patterns of the samples prepared with different surfactants. Fig. 2. TEM images of the samples prepared with different surfactants. (a) ZS1 ZS2

(c) ZS3

(b)

(d) ZS4

Fig. 3. Nitrogen adsorption-desorption isotherms and the corresponding density functional theory (DFT) pore size distributions (inset) of the samples prepared with different surfactants. (a) ZS1

(b) ZS2

(c) ZS3

(d) ZS4

Fig. 4. UV-vis absorption spectra of the samples prepared with different surfactants. Fig. 5. (a) Color removal of MO over different photocatalysts under UV irradiation; (b) Absorption spectra of MO aqueous solution in the presence of the sample ZS4 under UV irradiation at different time intervals. Fig. 6. XRD patterns of the samples prepared with different reactants. Fig. 7. TEM images of the samples prepared at different reactants. (a) ZS5

(b) ZS6

Fig. 8. Nitrogen adsorption-desorption isotherms and the corresponding density functional theory (DFT) pore size distributions (inset) of the samples prepared at different reactants. (a) ZS5

(b) ZS6

Fig. 9. UV-vis absorption spectra of the samples prepared at different reactants. Fig. 10. Color removal of MO over different photocatalysts under UV irradiation. Table 1 The BET specific surface areas of the samples prepared with different surfactants. Table 2 The BET specific surface areas of the samples prepared at different reactants. 23

Fig. 1

Fig. 2

24

Fig. 3

Fig. 4

25

Fig. 5

Fig. 6

Fig. 7

26

Fig. 8

Fig. 9

Fig. 10

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Table 1 The BET specific surface areas of the samples prepared with different surfactants. Samples

Specific surface area (m2·g-1)a

ZS1

117.2

ZS2

184.1

ZS3

189.1

ZS4

190.4

a. The BET specific surface area (SBET) was determined by the BET method.

Table 2 The BET specific surface areas of the samples prepared at different reactants. Samples

Specific surface area (m2·g-1)a

ZS5

30.7

ZS6

82.7

a. The BET specific surface area (SBET) was determined by the BET method.

28