Micro-emulsion-assisted synthesis of ZnS nanospheres andtheir photocatalytic activity

Yao Li, Xiaoyan He, Minhua Cao *

Department of Chemistry, Northeast Normal University, Changchun 130024, PR China

Received 27 May 2007; received in revised form 7 November 2007; accepted 10 November 2007

Available online 26 November 2007

Abstract

ZnS nanospheres with rough surface were synthesized by using a micro-emulsion-assisted solvothemal process. The molar ratioof [water]/[surfactant] played an important role in controlling the size of the ZnS nanospheres. X-ray powder diffraction (XRD),transmission electron microscopy (TEM), field emission-scanning electron microscope (FE-SEM), and selected area electrondiffraction (SAED) were used for the characterization of the resulting ZnS nanospheres. A possible formation mechanism wasproposed. These ZnS nanospheres exhibited a good photocatalytic activity for degradation of an aqueous p-nitrophenol solution andthe total organic carbon (TOC) of the degradation product has also been investigated.# 2007 Elsevier Ltd. All rights reserved.

Keywords: A. Chalcogénides; A. Nanostructures; A. Semiconductors; D. Catalytic properties

1. Introduction

In the past 20 years, nanosized materials have attracted considerable attention because of their technological andfundamental scientific importance and potential applications [1–8]. Most of these materials show unique electrical,optical, magnetic, and chemical properties, which cannot be achieved by their bulk counterparts. Generally, aboveproperties can be finely adjusted by controlling their size. Up to now, much work has been focused on the preparationand size- and shape-dependent property studies of semiconductor nanomaterials such as TiO2, ZnO, CeO2, Fe2O3,CdS, ZnS, and so on [9–14]. One important application of these materials is used as photocatalysts for the degradationof toxic chemicals. In particular, semiconductor materials with smaller dimensions often exhibit uniquephotoelectrochemical properties that arise from their quantum-size effect. Among abovementioned materials,TiO2 photocatalyst has been widely used to remove organic pollutants in environment protection [15–20].

As an important IIB–VIA group semiconductor material, nanosized ZnS has received considerable attention, due toits special functions in photocatalysis [21–23]. A photocatalytic process is based on electron–hole pairs created by theabsorption of photons by semiconductor materials, which can give rise to redox reactions with compounds absorbed onthe surface of a photocatalyst [21,24]. From the view of rapid generation of electron–hole pairs by photoexcitation andhighly negative reduction potential of excited electrons, ZnS nanomaterial is a good photocatalyst and has been used inthe photoreduction of water to produce hydrogen, the photoredution of CO2, and the photocatalytic degradation oforganic pollutants such as dyes, halogenated derivatives, and p-nitrophenol in waste water treatment [25–29]. Up to

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Materials Research Bulletin 43 (2008) 3100–3110

* Corresponding author.E-mail address: caomh043@nenu.edu.cn (M. Cao).

0025-5408/$ – see front matter # 2007 Elsevier Ltd. All rights reserved.doi:10.1016/j.materresbull.2007.11.016

now, various routes have been developed for the synthesis of ZnS nanocrystals (NCs) with different morphologies suchas quantum dots, nanorods, nanowires, nanospheres, and so on [30–35]. To fully achieve the photocatalysis propertiesof ZnS, well-defined structure is desirable.

Generally, as is known, the photoactivity of semiconductor materials can be intensively influenced by their structureand particle size, in which particle size is a particularly important factor. However, it is relatively difficult to control theparticle size of semiconductor materials within quantum size domain by using conventional synthesis methods. This ismainly due to that crystallites with smaller size possess high-surface energy and tend to aggregate or undergo Ostwaldripening to minimize their surface tension, resulting in the formation of larger particles [36,37]. In recent years, to obtainquantum-sized nanocrystals, several strategies have been successfully developed to prepare nanostructures with smallersize. One typical approach is the use of porous solid materials with microporous structure as microreactors to preventOstwald ripening by restraining molecular diffusion. Various porous materials such as carbon nanotubes [38], zeolites[39], layered compounds [36a,40], and porous anodic alumina membranes [41], have been used for synthesizingnanocrystals. Another universal method for synthesizing quantum-sized nanocrystals is the use of capping or stabilizingreagents, which can limit the crystal growth by adsorbing on the surface of nanocrystals. An alternative approach is to usesoft templates such as micelles to direct the formation of nanocrystals with quantum size. By comparison, porousmaterial-based synthesis is restricted by a number of disadvantages that include the tedious work involved in thepreparation and removal of the porous materials or the wide size distribution of obtained particles.

Here, we present the synthesis of novel ZnS nanospheres via micro-emulsion-directed method, which are self-assembled through the aggregation of small ZnS nanocrystals. The influence of molar ratio of [water]/[surfactant](defined as v) on the size of the final product was discussed. In addition, compared with other ZnS nanospheres, thesenanospheres possess rough surface, good dispersion, and high surface-to-volume ratio, which result in their high-photocatalytic activity. Recently, Bai and colleagues [12a] reported the synthesis of ZnS nanoporous nanoparticles,which were composed of building blocks comprising hexagonal wurtzite ZnS nanocrystals of several nanometers indiameter. The aggregation of these small ZnS nanocrystals leads to the formation of nanopores. Therefore, theypossess a specific surface area in the order of 156 m2 g�1, which results in an improved photocatalytic performancecompared with that of Degussa P25 titania or ZnS nanocrystals. Our as-synthesized ZnS nanospheres have samestructure as that reported by Bai and colleagues [12a]. Thus, it can be conjectured that when ZnS nanospheres with aslippery surface and larger size react with some pollutions, ZnS nanospheres only provide the limited surface for thereactant molecules. But the as-prepared ZnS nanospheres are different. The smaller size and rough surface of thenanocrystals could result in their high-surface area, which could provide more ideal adsorption sites for reactantmolecules. So it can be concluded that the as-synthesized ZnS nanospheres could be more active than other simple ZnSnanospheres. The subsequent photocatalytic property measurement further confirms the fact. p-Nitrophenol, as weknow, which has prominent water solubility and stability, is widely used in insecticides and herbicides, easily resultingin water pollution [17]. However, few studies on its degradation are reported especially using some otherphotocatalysts besides Degussa P25 titania [17]. In this paper, we report that these ZnS nanospheres can completelydegrade p-nitrophenol within 90 min under the ultraviolet (UV) light and the degradation ratio can reach more than97%, which is comparable to that of Degussa P25 titania.

2. Experimental

2.1. Materials

Cetyltrimethylammonium bromide (CTAB) was purchased from Shanghai Huishi Biochemical Reagent Co. Ltd.Cyclohexane and 1-pentanol were purchased from Tianjin Chemical Reagent Co. Ltd. zinc acetate and thiourea (Tu)were purchased from Beijng Beihua Fine Chemicals Co. Ltd. CTAB, cyclohexane, and 1-pentanol were used withoutfurther purification. Zinc acetate and thiourea (Tu) were of A.R.

2.2. Synthesis

Zinc acetate (Zn(CH3COO)2�2H2O) was used as starting material for the synthesis of ZnS nanospheres, andthiourea ((NH2)2CS) was used as the source of S2�. The micro-emulsion system consisted of cyclohexane as thecontinuous oil phase, CTAB as the surfactant, 1-pentanol as the co-surfactant, and an aqueous solution including zinc

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acetate and thiourea as the dispersed phase. In a typical synthesis, a solution containing 0.25 M Zn (CH3COO)2�2H2Oand 0.5 M (NH2)2CS was prepared in de-ionized water. This solution was dispersed in a mixture of 2.5 g CTAB, 60 mlcyclohexane, and 3 ml 1-pentanol to form a micro-emulsion. Afterward, this micro-emulsion was transferred into aTeflon-lined stainless steel autoclave, sealed, and maintained at 120 8C for 16 h. After the reaction was completed, theresulting product was collected by centrifuging, washed with absolute ethanol and distilled water several times, anddried in air atmosphere at room temperature.

2.3. Characterization

The size and morphology of the as-prepared products were characterized by transmission electron microscopy(TEM), which was carried out on the Hitachi model H-800 transmission electron microscope, and field emission-scanning electron microscopy (FE-SEM), which was carried out on the field-emission microscope (JEOL, 7500B)operated at an acceleration voltage of 200 kV. The microstructure of the as-obtained products was analyzed byselected-area electron diffraction (SAED) and high-resolution transmission electron microscopy (HRTEM) on a JEOLJEM-2010F transmission electron microscopy operated at 200 kV. The X-ray powder diffraction (XRD) image of theproduct was measured on a Rigaku X-ray diffractometer with Cu Ka radiation (l = 1.5418 nm) at a scan rate of 4 8/min in the range of 20–708.

3. Results and discussion

3.1. Morphology and structure of ZnS nanospheres

The micro-emulsion system used here is a thermodynamically stable mixture of oil, water, and surfactant. It is amacroscopically homogeneous mixture, but microscopically it consists of water or oil aggregates surrounded by amonolayer of surfactants [42,43d]. The water-in-oil micro-emulsion has been utilized for the formation of a largenumber of materials [43], though the mechanism of formation is still not understood in detail. Theoretically speaking,the diameter of the nanoparticles is directly proportional to the size of the micro-emulsion water pools, which isdetermined by the ratio of water-to-surfactant (v = [H2O]/[surfactant]). The water-to-surfactant molar ratios of lessthan 15 have the hydrodynamic diameter in the range of 4–10 nm and the molar ratios of greater than 15 have ahydrodynamic diameter range between 5 and 500 nm [43b]. Therefore, it can be deduced when the molar ratios aremore than 30, the hydrodynamic diameter may be closed to micron-scale.

In this paper, we used this reverse micelle-based method to synthesize the ZnS nanospheres. The size of the ZnSnanospheres can be adjusted by varying the v value. A series of experiments have been carried out by keeping theconcentration of the reactants constant (0.25 M Zn(CH3COO)2�2H2O and 0.5 M (NH2)2CS), but just changing the v

value from 8, to 16, 24, and 32. The morphology and size of the products have been investigated by TEM. It can be seenfrom Fig. 1a–d that all ZnS nanospheres have good uniformity. When the v value changed from 8, to 16, 24, and 32, theaverage diameter of these ZnS nanosphere samples changed from 200, to 250, 350, and 1500 nm, respectively, buttheir morphology kept unchanged. This result also further confirmed the change of hydrodynamic diameters of theresulting samples, in agreement with the molar ratio of water-to-surfactant. When the v value is kept at 8 and 16, ZnSnanospheres with average diameter of 200 and 250 nm were obtained, respectively. When the v value was increasedfrom 24 to 32, the diameter of particles can reach as large as 1500 nm. Considering this burst increase in diameter, thesize of the products could be controlled within nano-scale by adjusting the v value.

The phase composition and structure of as-synthesized products were examined by XRD. Because products withdifferent sizes all have same composition, we only show the XRD image of ZnS nanospheres with the averagediameter of 200 nm as an example. As shown in Fig. 1e, it is clear that the diffraction peaks appearing at 29.048,48.388, and 57.328 correspond to the (1 1 1), (2 2 0), and (3 1 1) planes of cubic ZnS phase with lattice constanta = 5.414 Å (JCPDS NO. 77-2100), and no other impurities have been detected in the product.

Fig. 2 presents the typical scanning electron microscope (SEM) images of ZnS nanospheres with diameter of200 nm. The low-magnification SEM image in Fig. 2a indicates that the sample is composed of a large quantity ofwell-tailored nanospheres. Fig. 2b shows a high-magnification SEM image for the same sample. It can be clearly seenthat the surface of the nanospheres is rough and constructed by many smaller particles. The ED pattern, as revealed ininset of Fig. 2c, exhibits diffuse rings instead of sharp spots, indicating that the ZnS nanospheres are polycrystalline.

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To investigate the microstructure of these rough spheres, high-resolution transmission electron microscopy (HRTEM)has been carried out. The HRTEM image in Fig. 2c further confirmed the fact that these ZnS nanospheres consist ofnanocrystals with small size.

3.2. Effects of the reaction conditions

To investigate the optimal reaction condition for the fabrication of ZnS nanospheres, a series of experiments havebeen carried out by changing the experimental parameters. It is found that the reaction temperature, time, and the

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Fig. 1. TEM images of ZnS nanoshperes with same concentration of the reactants but at different v values. (a) v = 8; (b) v = 16; (c) v = 24; (d)v = 32; (e) X-ray diffraction pattern of ZnS nanoshperes with average diameter of 200 nm.

concentration of the reactants are key factors for preparing ZnS nanospheres with perfect morphology and desirabledispersion. To obtain the influence of the reaction temperature on morphology and size of the product, the sampleswere synthesized at different temperatures. The temperature was changed from 90, 150, to 180 8C but keeping thesame reaction time (16 h). Fig. 3 presents the TEM images of the as-synthesized products obtained at three differenttemperatures. When the temperature was kept at 90 8C, Fig. 3a and b shows the TEM images of ZnS nanospheresobtained at v = 16 and 24, respectively. It can be seen that the ZnS nanospheres tended to aggregate together. Thesenanospheres are not uniform any more and the diameters are in the range of 80–250 nm for v = 16 and 100–300 nm forv = 24. When the temperature was increased to 150 8C, the typical morphologies of the products obtained at v = 16and 32, are shown in Fig. 3c and d. When the temperature was as high as 180 8C, besides the nanospheres with roughsurface and diameter from 1.25 to 3 mm, smooth inhomogeneous particles with diameter of 10–30 nm, long belts, andbig sheets (Fig. 3e and f) also existed in sample. Compared all these results with the ZnS nanospheres obtained at120 8C as shown in Fig. 1, it can be concluded that 120 8C is the optimal temperature for the formation of well-definednanospheres.

To find the suitable reaction time, we fixed the reaction temperature at 120 8C and molar ratio of [water]/[surfactant] at 16. Fig. 4 shows the typical results obtained at different times. When the reaction time was maintained at4 h, nanospheres cannot be formed, completely similar to those for reaction time of 8 h (Fig. 4a and b). When thereaction time was increased to 24 h, the formed spheres were still non-uniform (Fig. 4c), although these nanospheres

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Fig. 2. (a) Low-magnification SEM image of ZnS nanoshperes; (b) high-magnification SEM image of ZnS nanoshperes; (c) electron diffractionpattern; (d) HRTEM image of ZnS nanoshperes.

can be formed. Compared these TEM figures with Fig. 1, which has uniform size and good dispersion, it is found thatthe optimized reaction time for the formation of nanospheres is 16 h.

Much more studies indicate that in addition to the reaction temperature and time, the concentration of the reactantshas important influence on the morphology of the products. Fig. 5 presents TEM images of ZnS nanostructuresobtained at 120 8C for 16 h, but with different concentrations of the reactants. As shown in Fig. 5a and b, the ZnSsamples were synthesized using a concentration of Zn(CH3COO)2�2H2O of 0.125 M and (NH2)2CS of 0.5 M with thevalue of v = 16 and 24, respectively. It can be seen that the ZnS sample tends to aggregate and has an irregular shape.When the concentration of Zn(CH3COO)2�2H2O increased to 0.25 M (v = 8, 16, 24, and 32) and 0.5 M (v = 16) withthe concentration of (NH2)2CS still at 0.5 M, the TEM images were shown in Fig. 1 and Fig. 5c. It is clear that when theconcentration of Zn(CH3COO)2�2H2O is 0.25 M, the product are well-defined nanospheres. When the concentrationof Zn(CH3COO)2�2H2O was 0.5 M, the diameters of the ZnS nanospheres ranged from 500 nm to 1.25 mm.

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Fig. 3. TEM images of ZnS samples obtained at 90 8C (a) v = 16, (b) v = 32; TEM images of ZnS samples obtained at 150 8C (c) v = 16 (d) v = 32;TEM images of ZnS samples obtained at 1808 (e) v = 16, (f) v = 32.

In above work, all samples were obtained based on the procedure that an aqueous solution including zinc acetateand thiourea was dropwise dropped into one solution containing CTAB, 1-pentanol, and cyclohexane, and then theresulting micro-emulsion was solvothermally treated. We also had prepared two micro-emulsion systems containingtwo reactants, respectively. Then, the two micromulsion systems were completely mixed to form one micromulsionsystem. After solvothermal treatment of this micro-emulsion system, the product was formed. Fig. 5d shows the TEMimage of the sample (v = 16), which was synthesized by using the above procedure. Compared with Fig. 1c, it isevident that the morphology of nanospheres is not good enough.

3.3. Possible formation mechanism of ZnS nanospheres

Based on the above results, an aggregation mechanism for the formation of ZnS nanospheres with rough surface isproposed. In our experiment, zinc acetate (Zn(CH3COO)2�2H2O) and thiourea ((NH2)2CS) were chosen as reactants.When zinc acetate dissolves in water, it can release Zn2+ directly. However, the thiourea should be decomposed underhigher temperature conditions to release S2� ions, and then these S2� ions react with Zn2+ ions to form the ZnSmonomers that will grow into ZnS nanocrystals. The nucleation of ZnS is well known to be very fast, and at the sametime, the small ZnS nanocrystals formed in the nanosize water pools have a higher surface energy. Driven by theminimization of interfacial energy, the ZnS nanospheres were formed through the self-assembly aggregation of theZnS primary nanocrystals. This growth process including two steps can be illustrated in Fig. 6. This kind ofaggregation belongs to non-oriented attachment, which is confirmed by the HRTEM image.

3.4. Photocatalytic activity

Semiconductors have a void energy region, which extends from the top of the filled valence band to the bottom ofthe vacant conduction band called the band gap. The basic principle of semiconductor photocatalysis involves the

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Fig. 4. TEM images of the ZnS samples prepared with different reaction times, (a) 4 h; (b) 8 h; (c) 24 h.

absorption of a photon by semiconducting solids and the excitation of an electron (e�) from the valence band to theconduction band. Simultaneously, in the valence band an electron vacancy or a positive charge called a hole (h+) isgenerated (hn + semiconductor! h+ + e�). The electron–hole pair (e�–h+ pair) migrates to the surface of thephotocatalyst, where it could either recombine, producing thermal energy, or reacts with the adsorbed compounds,

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Fig. 5. TEM images of the as-synthesized ZnS samples with 0.5 M (NH2)2CS and 0.125 M Zn(CH3COO)2�2H2O (a) v = 16; (b) v = 32; (c) 0.5 MZn(CH3COO)2�2H2O v = 16; (d) TEM image of the ZnS sample synthesized by mixing the two micro-emulsions.

Fig. 6. Schematic representation of the formation mechanism of the ZnS nanospheres.

leading to the degradation of pollutants. Generally, to improve the efficiency of the catalysis, increasing the surfacecharge transfer rate and reducing the e�–h+ recombination rate are important [24]. The as-prepared ZnS nanospheresbecause of their rough surface and small size possess higher surface areas, which could provide more active surfacesites. On these sites surface charge could transfer fast, which reduces the e�–h+ recombination rate.

The size and surface conditions of a photocatalyst are important parameters since they could directly influence theresult of catalysis. In our work, we demonstrated the degradation of p-nitrophenol using the as-synthesized ZnSnanospheres as catalyst, which is relatively difficult to degrade in aqueous media [17]. As shown in Fig. 7a, the ZnSnanospheres are very active for the degradation of p-nitrophenol under UV exposure ([p-nitrophenol] = 25 ppm,100 ml dispersion, and 180 mg of ZnS nanospheres with diameter of 200 nm loading). It is clearly seen that the ZnSnanospheres show nearly complete degradation of p-nitrophenol within 90 min. Even in the process of adsorptionthese nanospheres could degrade p-nitrophenol up to 12%, and in the first 10 min under UV irradiation, the ratio ofdegradation could grow up to 40%. Degussa P25 titania as a commercial photocatalyst is usually used in degradationof organics for its unusual catalysis function. To demonstrate the photocatalytic activity of the present ZnSnanospheres, we used Degussa P25 titania to degrade p-nitrophenol under UV light under the same conditions ([p-nitrophenol] = 25 ppm, 100 ml dispersion, and 180 mg of P25 titania loading) for comparison. The result was shown

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Fig. 7. Photodegradation image of p-nitrophenol (25 ppm and 100 mL) (a) with ZnS nanospheres; (b) with Degussa P25 titania under UV light. Ct

and C0 stand for the p-nitrophenol concentration after and before irradiation, respectively.

Fig. 8. The TOC of the degradation result of p-nitrophenol (a) with ZnS nanospheres and (b) with Degussa P25 titania. Ct and C0 stand for the p-nitrophenol concentration after and before irradiation.

in Fig. 7b. It can be seen that the degradation ability of the ZnS nanospheres was comparable to that of Degussa P25titania. So, it can be concluded that the excellent catalytic properties of the as-synthesized ZnS nanospheres are relatedto their rough surface and small size. So these nanocrystal-assembled ZnS nanospheres could act as an activephotocatalyst.

The total organic carbon (TOC) of degradation productions of p-nitrophenol has also been detected. As shown inFig. 8, it can be seen that degradation of TOC in aqueous p-nitrophenol solution by the ZnS nanosphere sample andDegussa P25 titania reach 53% (Fig. 8a) and 51% (Fig. 8b) within 120 min, respectively. This result further confirmsthat the degradation ability of ZnS nanospheres was closed to that of Degussa P25 titania.

4. Conclusion

In summary, ZnS nanospheres self-assembled from ZnS nanocrystals have been achieved by using a facile micro-emulsion system at a low temperature. By carefully controlling the experimental parameters including the reactiontemperature, time, the concentration of the reactants, the v value, and so on, uniform ZnS nanospheres with tunablesize (200–1500 nm) have been successfully obtained. These nanospheres, possessing rough surface and high surface-to-volume ratio, provide more active reaction sites and exhibit effective photocatalytic properties, as demonstrated inthe photodegradation of aqueous p-nitrophenol solution. Because of their distinct photocatalysis, we expect thesespecial ZnS nanospheres could represent a good candidate for further applications in environmental remediation.

Acknowledgments

The author thanks the National Natural Science Foundation of China (NSFC, 20401005 and 20771022), the JilinDistinguished Young Scholars Program Foundation, the Huo Yingdong Foundation for financial support, and analysisand test fund of Northeast Normal University.

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