Physica E 43 (2011) 1071–1075

Contents lists available at ScienceDirect

Physica E

1386-94

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/physe

ZnS nanoparticles self-assembled from ultrafine particlesand their highly photocatalytic activity

Xiaoyan Li a, Chenguo Hu a,n, Hong Liu b, Jing Xu a, Buyong Wan a, Xue Wang a

a Department of Applied Physics, Chongqing University, Chongqing 400044, PR Chinab State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, PR China

a r t i c l e i n f o

Article history:

Received 15 October 2010

Received in revised form

14 December 2010

Accepted 4 January 2011Available online 19 January 2011

77/$ - see front matter & 2011 Elsevier B.V. A

016/j.physe.2011.01.001

esponding author. Tel.: +86 23 65104741; fa

ail address: hucg@cqu.edu.cn (C. Hu).

a b s t r a c t

ZnS nanoparticles self-assembled from ultrafine particles (2–6 nm) have been synthesized for the first

time by the composite molten salt method. UV–vis reflectance spectrum and photoluminescence of the

ZnS nanoparticles were investigated. The narrowed band gap of the ZnS nanoparticles was found owing

to novel crystal structure formed from a lattice-matched ‘coherent interface’ of the agglomerated

ultrafine particles. Photocatalytic degradation of methyl orange by the nanoparticles shows much

better photocatalysis than that by the commercial ZnS powder under simulated sunlight irradiation.

& 2011 Elsevier B.V. All rights reserved.

1. Introduction

Due to their unique physical and chemical properties, II–VIsemiconductor materials have attracted great interest in the lastdecade [1–3]. ZnS is an important II–VI semiconductor with a largeband gap of 3.6–3.8 eV [4], which is considered as one of the mostuseful functional materials in the electroluminescent devices [5],nano-generator [6], gas sensors [7], biological fluorescence materi-als [8] as well as photo-degradation catalysts. It is known thatsemiconductors under illumination with photon energy larger thantheir band gap can produce electron–hole pairs. Because some ofthese electron–hole pairs diffuse out to the surface of the crystaland take part in chemical reactions with donors or acceptors fromsurroundings, a photocatalytic reaction is achieved. ZnS nanocrys-tals could be used as a good photo-catalyst due to rapid generationof the electron–hole pairs by photo-excitation and the highlynegative reduction potentials of excited electrons, as conductionband position of ZnS in aqueous solution is higher than that ofother semiconductor such as TiO2 and ZnO [9]. It has beendemonstrated by many reports that ZnS nanocrystals have thecatalytic degradation capability for some organic pollutants inwater [10,11]. Since a larger ratio of surface to volume of a catalystwould facilitate a better catalytic activity [11,12], the size con-trolled synthesis of ZnS nanostructures to produce a larger ratioof surface to volume is of great importance. In the past decade,semiconductor micro/nanoparticles have been synthesizedthrough various ways including hydrothermal process [13],micro-emulsion method [14], sol–gel method [15], chemical co-

ll rights reserved.

x: +86 23 65105925.

precipitation method [16], etc. However, these methods normallyconsist of two or more steps, and rigorous conditions, such as highpressure or high temperature, are usually required [10,13,14].

In this paper, we have adopted the composite molten saltmethod (CMS) for synthesis of ZnS nanoparticles. The compositemolten salt method is a new strategy for synthesizing nanostruc-tures, which has advantages of being one-step, easy scale-up,low cost and environmentally friendly [17,18]. The synthesis ofZnS nanoparticles is based on the reaction between a metallicsalt and metallic sulfide in a solution of eutectic composite saltsat a temperature of 200 1C and ambient pressure, without usingorganic dispersant or capping agent. The optical properties of theZnS nanoparticles are characterized by UV–vis reflection, photo-luminescent (PL) spectrum. To explore possible application of theZnS nanoparticles, the catalytic degradation of methyl orange iscarried out under illumination of simulated sunlight.

2. Experimental

2.1. Synthesis of ZnS nanoparticles

All chemicals were analytically pure and were purchased fromChongqing Chemical Company. In a typical synthesis process, a9 g mixture of LiNO3 and KNO3 at a mass ratio of 1/2 was placedin a 25 mL Teflon vessel. 0.305 g Zn(NO3)2 �6 H2O and 0.246 gNaS �9 H2O were put in the vessel with 1 mL NH3 �H2O in themixture to keep a weak base reaction environment. The vesselwas then placed in the furnace, which had been preheated to200 1C. After reacting for 72 h, the vessel was taken out andallowed to cool to room temperature. The solid product wasdissolved by deionized water, and then filtered and washed with

X. Li et al. / Physica E 43 (2011) 1071–10751072

deionized water for several times. Finally, the white powder wasdried at 60 1C.

2.2. Characterization

X-ray diffraction (XRD, BDX3200, China) and energy dispersiveX-ray spectroscopy (EDS) were used to investigate the crystallinephase and chemical composition of the samples. Transmissionelectron microscopy (TEM, Hitachi H-800) and high resolutiontransmission electron microscopy (HRTEM, JEOL-2100) were usedto characterize the morphology, size and crystalline degree of theas-synthesized product. The morphology of the commercial ZnSpowder was characterized by scanning electron microscopy (SEM,TESCAN VEGA2).

To detect the optical property, the optical reflectance spectrumof the product was measured by an UV–vis-NIR spectrophot-ometer (Hitachi U-4100) on a film made from the ZnS nanopar-ticles on quartz slices. Briefly, the ZnS nanoparticles were firstdispersed in the alcohol to form a suspension. Then, the suspen-sion was deposited onto the quartz substrate layer by layer underirradiation of an infrared lamp at 80 1C to form a thin film. PLspectrum was examined on the dispersed ZnS particles in ethanolby SPEX Flurolog-2 spectrofluorometer at room temperature withan excitation wavelength of 280 nm.

2.3. Degradation of methyl orange

The degradation of methyl orange was carried out by asimulated sunlight instrument (CHF-XM-500W). A 20 mg sampleof ZnS nanoparticles was dispersed in a 100 mL solution of16.4 mg/L methyl orange in a glass beaker. Then, the solutionwas stirred by the magnetic agitator under the simulated sunlightwith an intensity of 90 mW/cm2. The solution for characterizationof degradation was subtracted from the beaker every hour andcentrifuged to remove the particles from the solution. In compar-ison, analogous experiments were performed with commercialZnS powder under the same conditions.

3. Results and discussions

3.1. Characterization

Fig. 1a shows the XRD pattern of the sample synthesized at200 1C for 72 h. All the XRD diffraction peaks of ZnS nanoparticles

0200

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16,2

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1

11

tyen

sit

Inte

A

20 30 40 50 60 70 802theta (degree)

Fig. 1. (a) XRD pattern of the commercial ZnS powder (line A) and t

(line B) can be assigned to a pure hexagonal phase of ZnS (JCPDS-89-2739). The commercial ZnS powder (line A) has the samepeaks as the sample of the ZnS nanoparticles. EDS measurementsin Fig. 1b prove that the elements in the nanoparticles are zincand sulfide. The TEM image and HRTEM image displays the sizeand morphology of the sample. We can see particles with thediameter of 10–20 nm in Fig. 2a and the magnified selected areain Fig. 2b. To clearly see the details, a higher magnified image isprovided in Fig. 2c, from which we can see that the particles areassembled with many ultrafine particles, as marked in arrows.HRTEM image in Fig. 2d further demonstrates the assemblybehavior of the ultrafine particles. The ultrafine particles arecrystals with diameter of 2–6 nm (marked by circles in Fig. 2d)and their polygonal shapes tend to share common faces toassemble into bigger particles. Agglomeration of nanoparticles isa common phenomenon, as nanoparticles tend to decrease theexposed surface in order to lower the surface energy, and thesmaller particle size results in stronger agglomeration and for-mation of a lattice-matched ‘coherent interface’ [19,20]. HRTEMimage and Fast Fourier transform (FFT) in Fig. 2d illustrate thelattice-matched ‘coherent interface’ and the fake single-crystalstructure of the agglomerated nanoparticles. We think that themolten composite and molten salts provide a high viscositysolvent, which prevents the nanoparticles from agglomeratedgrowth in the synthesis process. The ultrafine particles assembleinto bigger particles with size of 10–20 nm after they werewashed from the composite salt by deionized water and can onlybe dispersed again in aqueous solution by a vigorous ultrasonica-tion, but will remain as assembled particles under a mild stirringor deposition on a substrate.

Fig. 3a shows the UV–vis reflection spectrum of the ZnSnanoparticles. There is a steep absorption edge at about 350 nm.The Eg of ZnS nanoparticles can be further confirmed by extra-polating the linear part of Kubelka–Munkfunction (Fig. 3a), whichis the ratio of absorption coefficient to scattering factor from theoptical diffuse reflectance spectrum. The band gap (Eg) is 3.55 eV,a little smaller compared with that of the band gap for bulkwurtzite ZnS (3.8 eV), which is contradictory to the quantum sizeeffect. However, the lattice-matched ‘coherent interface’ fromthe agglomerated nanoparticles (Fig. 2d) prevents the detectionof quantum size effect. In addition, some vacancies in the fakesingle-crystal structure of the spherical particles, as marked byarrows in Fig. 2d, might induce sublevels formed within the bandgap. The PL spectrum taken from the dispersed ZnS nanoparticlesin ethanol solution is shown in Fig. 3b. It displays an obvious

keV

he ZnS nanoparticles (line B). (b) EDS of the ZnS nanoparticles.

100 nm

100 nm

20 nm55 nm

Fig. 2. Low magnified TEM image of the ZnS nanoparticles (a), magnified (b) and higher magnified (c) image of selected area in (a). HRTEM image (d) and FFT (inset) of the

ZnS nanoparticles.

Fig. 3. (a) UV–vis reflection spectrum and Kubelka–Munk function of the ZnS nanoparticles. (b) PL spectrum of the ZnS nanoparticles at room temperature with an

excitation wavelength of 280 nm.

X. Li et al. / Physica E 43 (2011) 1071–1075 1073

emission peak centered at 366 nm corresponding to the bandedge emission, and the broad peak centered at 467 nm corre-sponding to the transitions involving sulfur vacancy states in ZnSnanoparticles [21,22].

3.2. Degradation of methyl orange

To explore the potential application of the ZnS nanoparticles,we carried out the degradation experiment of methyl orange.

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Fig. 4. Absorption spectrum of 16.4 mg/L methyl orange solution with 0.2 g/L ZnS nanoparticles in different stages under illumination of the simulated sunlight (90 mW/cm2)

(a) and in the dark (b). Absorption spectrum of the same starting solution with 0.2 g/L commercial ZnS powder under the same illumination (c) and the SEM image of the

powder (inset). The plots of the absorption peak intensity of degraded methyl orange versus time (d).

VB

CB

h

h

ee

hν

O2-O2

O2 O2-

OH-/H2O ·OH

Scheme 1. Schematic diagram showing the agglomerated ZnS ultrafine particles

and the mechanism of photocatalytic degradation of methyl orange.

X. Li et al. / Physica E 43 (2011) 1071–10751074

The absorption peak of methyl orange centered at 460 nm ismonitored to characterize the degradation effect. The commoncatalytic degradation contains two aspects, adsorption ability andphotocatalytic degradation ability. Fig. 4a and b shows the UV–visabsorption spectrum of the starting solution (16.4 mg/L methylorange) and the solution with 0.2 g/L ZnS nanoparticles forcatalytic degradation at different stages under the simulatedsunlight (90 mW/cm2) and in the dark, respectively. It can beseen that the concentration of the methyl orange decreases withillumination time under the simulated sunlight. However we cansee a small change in the solution in the dark condition. We canconclude that the degradation is mainly caused by photocatalysisother than adsorption. Similar experiment with the commercialZnS powder, which consists of the particles with diameter up toseveral micrometers (inset Fig. 4c) under the same conditions,was carried out and the result indicates much weaker catalyticability, as shown in Fig. 4c. Fig. 4d shows the plots of theabsorption peak intensity of the degraded methyl orange versustime. The slopes of the lines indicate different degradation ratesby the catalysis of the samples. After 5 h illumination, about 70%and 18.2% of methyl orange can be effectively degraded by thecatalysis of the ZnS nanoparticles and the commercial ZnSpowder, respectively.

Photocatalytic reaction mechanism is proposed as follows:electron (e�) and hole (h+) pairs of the ZnS nanoparticles aregenerated under the light excitation (absorbed energy equal to or

larger than its band gap). Then, these electrons and holes reactwith the adsorbed surface substances, like O2, OH� , etc., to formreactive species O2

� , dOH, and then these reactive species degradethe methyl orange into the small molecules like CO2, H2O,etc. [23,24]. Schematic diagram in Scheme 1 shows the agglom-erated ZnS ultrafine particles and the mechanism of photocat-alytic degradation of methyl orange. The effective catalyticdegradation of methyl orange by the ZnS nanoparticles resultsfrom its unique assembled structure and its narrowed band gap,which would provide a tremendous specific surface area for

X. Li et al. / Physica E 43 (2011) 1071–1075 1075

contact of methyl orange and more effective absorption sunlightthan that of common nanoparticles.

4. Conclusions

ZnS nanoparticles (10–20 nm) self-assembled from 2–6 nmparticles have been obtained for the first time using a CMSapproach. The method is novel, one-step, template-free, low cost,nontoxic and mass-producible. The band gap of the ZnS nano-particles is narrowed compared with that of the band gap for bulkwurtzite ZnS, which is attributed to some vacancies in the fakesingle-crystal structure formed from a lattice-matched ‘coherentinterface’ of the agglomerated ultrafine particles. The photocata-lytic degradation on methyl orange illustrates that this newlysynthesized ZnS nanoparticles could be used as a promisingcatalytic degradation material for the organic pollutant removal.

Acknowledgments

This work is supported by the NSFC (60976055), Postgraduates’Science and Innovation Fund (201005B1A0010339) and InnovativeTraining Project (S-09109) of the 3rd-211 Project, and the large-scale equipment sharing fund of Chongqing University.

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