preparation and characterization of zinc sulfide nanoparticles under high-gravity environment

Preparation and characterization of zinc sulfide nanoparticlesunder high-gravity environment

Jianfeng Chena,*, Yaling Lia, Yuhong Wanga, Jimmy Yunb, Dapeng Caob

aResearch Center of Ministry of Education for High Gravity Engineering and Technology,

Beijing University of Chemical Technology, Beijing 100029, ChinabNanoMaterials Technology Pte Ltd., Blk 26 Ayer Rajah Crescent #07-02,

Singapore 139944, Singapore

Accepted 12 October 2003

Abstract

Nanosized ZnS particles were prepared under high-gravity environment generated by the rotating packed bed

reactor (RPBR) using zinc nitrate solution and hydrogen sulfide gas as raw materials. The effects of experimental

conditions such as reactant concentration, reaction temperature, rotating speed of the RPBR and aging time, on

the preparation of nanosized ZnS particles were investigated. A set of suitable operating parameters (the aging

time of 48 h, concentration of zinc nitrate of 0.1 mol/l, reaction temperature of 45 8C and rotating speed of the

RPBR of 1500–1800 rotation/min) for the preparation of nanosized ZnS were recommended. Under these

optimum conditions, well-dispersed ZnS nanoparticles was obtained. The crystal structure, optical properties,

size and morphology of the product were also characterized by XRD, UV-Vis spectrophotometer, and TEM,

respectively. Results indicate that the prepared ZnS has a good absorption for light in the wavelength range of

200–330 nm. XRD analysis also shows the prepared ZnS is in a sphalerite crystal phase. The process has great

potential of commercialization.

# 2003 Elsevier Ltd. All rights reserved.

Keywords: A. Nanostructures; B. Chemical synthesis; C. High gravity; D. Optical materials

1. Introduction

In recent years, the preparation and characterization of chalcogenides of different groups haveattracted considerable attention due to their important nonlinear properties, luminescent properties andother important physical and chemical properties [1]. Among these materials, ZnS is one such II-VIsemiconductor with direct wide band gap (3.6 eV), which has wide ranging applications in solar cells,

Materials Research Bulletin 39 (2004) 185–194

* Corresponding author. Tel.: þ86-10-6446466; fax: þ86-10-64434784.

E-mail address: [email protected] (J. Chen).

0025-5408/$ – see front matter # 2003 Elsevier Ltd. All rights reserved.

doi:10.1016/j.materresbull.2003.10.017

infrared window materials, photodiode and cathode-ray tube [2], electro luminescent devices andmultiplayer dielectric filters [3]. Therefore, much research on ZnS has been carried out [4–17].

Many different methods were used to prepare zinc sulfide particles [1,4–11] and thin membrane ofzinc sulfide [12–17]. Zhang et al. [4] prepared the ZnS nano-rods in lamellar liquid crystals of C12E4 bymixing zinc ions and thioacetamide (TAA) solution. They also investigated the effects of the reactantconcentration and the surfactant/water molar ratio in the liquid crystal system on the morphology andsize of the ZnS particles. Sanchez-Lopez et al. [7,8] used the gas-phase condensation method (alsocalled inert gas evaporation) to prepare quantum-size ZnS nanoparticles. In the gas-phase condensationmethod, the formation of the particles proceeded by condensation of the vapor material, nucleation andgrowth. In addition, ZnS is also produced by various high-vacuum techniques such as molecular beamepitaxy and chemical vapor deposition [11,18]. These techniques involve the transport of precursorvapors to a heated substrate and produce films of exceptional quality, but they are expensive to use.Therefore, Donahue et al. [11] used a more cost-effective alternative, sol-gel processing, to replacevacuum techniques for the preparation of ZnS. Recently, irradiation method was also extensively usedto prepare the ZnS nanoparticles [1,5]. For preparation of ZnS thin film, Torimoto et al. [12] usedelectrochemical atomic layer epitaxy technique, while Pawaskar et al. [15] used liquid–liquid interfacereaction technique. The ion plating method [13] was also used to prepare the triboluminescent ZnSfilm. In the investigation mentioned above, the size of zinc sulfide prepared was often comparativelylarge, and the cost was also relatively high. More importantly, it is difficult to commercialize thesepreparation processes due to the high-cost and low-volume capacity, and so they are still at theexperimental stage. Therefore, it is important to find a new method or process which can producesmaller sized ZnS nanoparticles at lower cost. Chen et al. [19] developed a new technology called high-gravity reactive precipitation to synthesize successfully various kind of nanoparticles including calciumcarbonate, Al(OH)3 and BaTiO3, etc., and a commercial production line of annual capacity of 10,000tones to manufacture nanosized CaCO3 at average particles size of 15–30 nm has been operatedsuccessfully since the year 2001. The commercial operation experience demonstrates the advantages oflow cost and high volume of this technology. In the new technology, Chen et al. used rotating packedbed reactor (RPBR) to simulate the high-gravity environmental condition to prepare nanoparticles.

In this work, we report the preparation of nanosized ZnS particle using this high-gravity technique.From a series of experiment, a set of optimized operating parameters was recommended for the reactiveprecipitation of the well-dispersed nanosized ZnS. The ZnS nanoparticles as-prepared werecharacterized by using transmission electron microscopy (TEM) and X-ray diffraction (XRD).

2. Experiment

The process diagram for the preparation of nanosized zinc sulfide particles in the RPBR is shown inFig. 1.

The zinc nitrate solution with the concentration of 0.1 mol/l was prepared and fed into the stirred tank1. Then, the solution was pumped by pump 10 into the RPBR through the liquid distributor andcirculates through the RPBR in the system during the entire reaction. Nitric acid was simultaneouslyused to adjust the initial pH value of the solution to 3.0. H2S gas was released into the RPBR from thegas cylinder and reacted with Zinc Nitrate solution in the packing section of RPBR to yield ZnSparticles. Details of structure of the RPBR and process operation could be referred to our previous

186 J. Chen et al. / Materials Research Bulletin 39 (2004) 185–194

paper (Chen et al. [19]). The process of reaction was tracked by using pH meter (a pHS-25 acidity-meter) to determine the end point of reaction. An alkali solution was used to absorb tail gas leaving theRPBR.

The morphology and sizes of the particles prepared in the RPBR were determined by transmissionelectron microscopy (TEM). The TEM images were taken with a Hitachi Model H-800 TransmissionElectron Microscope with an accelerating voltage of 200 kV. The powder XRD patterns were recordedusing a XD-3A Cu Ka X-ray diffractometer produced by Shimadzu Company of Japan. UV-Visabsorption spectra were measured by a Shimadzu UV-2501 PC spectrometer.

3. Results and discussion

In the process of the reaction, the curve of pH value changing with time tracked by pH meter isshown in Fig. 2. Since the reaction for the synthesis of ZnS is irreversible, the slope of pH valuechanging with reaction time represents the reaction rate. It can be found from Fig. 2 that pH valuedecreases sharply in 1 min after the commencement of the reaction. Then, the decrease levels off. Theresult demonstrates that this is an instantaneous reaction and the synthesis of zinc sulfide was finishedin a very short time.

3.1. The effects of operation parameters on the characteristics of ZnS nanoparticles

3.1.1. Effect of aging timeThe reaction of zinc nitrate and H2S is very rapid intrinsically. Moreover, the mass transfer

coefficient between gas and liquid interfacial phases under the high-gravity condition in RPBR isintensified 1–3 magnitude order larger compared to that in atmospheric condition. Therefore, thisreaction was completed in about 2–4 min. Formation of any crystallites needs two processes ofnucleation and growth. Due to the explosion like speed of the nucleation of nanosized zinc sulfide in theRPBR, further aging process is adopted to grow the particles. We used the solution of zinc nitrate (A.R.grade) with the concentration of 0.1 mol/l and the initial pH value 3.0, and volumetric ratio of 1:1 ofH2S gas (purity 99.9%) and the solution to prepare the ZnS particles at T ¼ 35 8C. The TEMs of ZnS

Fig. 1. The process diagram for the preparation of nanosized zinc sulfide in the RPBR. (1) stirred tank, (2) frequency

modulator, (3) motor, (4) gas inlet, (5) gas outlet, (6) liquid inlet, (7) sample outlet, (8) liquid flowmeter, (9, 12, 15, 17) valve,

(10) pump, (11) gas flowmeter, (13) cylinder, (14) suspension outlet, (16) absorption cell, (18) product outlet, (19) RPB

reactor.

J. Chen et al. / Materials Research Bulletin 39 (2004) 185–194 187

nanoparticles prepared changing with aging time in the RPBR at the rotating speed of 1500 rotation/min are shown in Fig. 3. It can be seen that aging time has a significant effect on the shape of particles.The shape of the product obtained just at the end of the reaction is irregular and the small and largeparticles co-exist. Larger particle tends to be spherical shape, while smaller particle forms anagglomeration of particles. With the aging process proceeded, the particles become regular spheres.With the increase of aging time, the size of the crystal particles increases. After aging for 24 h, theparticles become spherical but the particles are not well-dispersed. After aging for 48 h, a well-dispersed product with an average size of 23.2 nm was obtained, and the particle size distribution is alsonarrow. Further increase in aging time does not affect the product shape and size. Therefore, 48 h wasconsidered to be the optimum aging time.

3.1.2. Effect of concentrationUnder conditions of initial pH value 3.0, T ¼ 25 8C and rotating speed of 1500 rotation/min, we

investigated the effect of three reactant concentrations (0.1, 0.24, and 0.34 M) on product size afteraging for 48 h. Results indicate that particle size decreases with the increase in concentration. For thezinc nitrate solution with the concentration of 0.1 mol/l, product average size is 23.4 nm, and particlesize distribution is very narrow with 95% of the particles distributing in the range of 15–30 nm.

Based on Gibbs–Thomson equation, critical crystal nucleus size, rc is given by

rc ¼2Vs

RT ln S

where R is gas constant, T is Kelvin’s temperature, S is supersaturation, s is the surface energy. Theincrease of reactant concentration raises the supersaturation and therefore reduces the critical crystalnucleus radius. According to homogeneous nucleation theory, nucleation rate, J, is expressed as

J ¼ O exp�A0

ln2 S

� �

where O is the effective collision time. J is very sensitive to supersaturation S. With the increase ofsupersaturation contributed by increasing reactant concentration, nucleation rate of ZnS rapidlyincreases, which make product size reduce because of the formation of more crystal nucleus. Inaddition, the conversion of zinc ion was measured by chemical titration, and shown in Fig. 4. It can be

Fig. 2. pH values changed with reaction time.


Fig. 4. Conversion changed with concentration of reactant.

Fig. 3. TEM photos of ZnS at different aging time: (a) t ¼ 0, (b) t ¼ 24 h, (c) t ¼ 48 h, (d) t ¼ 96 h.


observed that the increase of reactant concentration causes the rapid reduction of conversion of zincion. Conversion of zinc ion can reach 97% at the reactant concentration of 0.1 M while it is 70% at theconcentration of 0.2 M. In the reaction, the increase of reactant concentration causes the concentrationof nitric acid to increase, which reduces the solubility of H2S. Therefore the conversion of zinc iondecreases with the increase of reactant concentration. So while the increase of reactant concentrationcan reduce particle size, it is achieved at the expense of conversion. Accordingly, with thisconsideration, the reactant concentration of 0.1 mol/l was recommended.

3.1.3. Effect of temperatureWe investigated the effect of temperature on particle size of ZnS and on the conversion of zinc ion

under the condition of other parameters keeping unchanged. In this work, four temperatures of 20, 35,45, 50 8C were chosen. The effect of temperature on particle size and conversion was shown in Fig. 5. Itcan be observed that with the increase of temperature, particle size decreases to a minimum value atT ¼ 45 8C and then increases, while conversion increases to a maximum value at T ¼ 45 8C and thenreduces. Therefore, at T ¼ 45 8C, we can get smaller particle size and higher conversion of zinc ion. Asa result, 45 8C is believed an optimum reaction temperature.

3.1.4. Effect of rotating speedThe effect of rotating speed on ZnS size and conversion of zinc ion was investigated. A comparison

of effect of five different rotating speed (900, 1200, 1500, 1800, 2100 rotation/min) on particle size isshown in Fig. 6. It can be observed that particle size reduces to a minimum value and then increaseswith rotating speed. In the RPBR, the centrifugal force produced by the rotation of rotor at high speedscauses the solution to break down into numerous small droplets within the packing section. The highdispersion and drastic onflow of liquid phase in the RPBR maintains a high concentration of reactant atthe gas–liquid interface, which greatly enhances gas–liquid mass transfer. The increase of rotatingspeed accelerates the renewing of liquid interface and reduces particle size. Fig. 7 shows the change inconversion with rotating speed. We can see little change of the conversion of zinc ion in the range of800 rotation/min to 1800 rotation/min. However, when the rotating rate increases to 2100 rotation/minfrom 1800 rotation/min, the conversion undergoes a steep reduction. Based on the analysis above, werecommended a rotating rate of 1500–1800 rotation/min.

Fig. 5. (1) Particle size changed with temperature. (2) Conversion changed with temperature.


3.2. Characterization of the ZnS prepared

Under the optimum conditions discussed previously, the prepared suspension of ZnS particles wasaged, filtered, washed and dried. The UV absorption characteristics of the prepared ZnS were measuredby a Shimadzu UV-2501 PC spectrometer, and the powder XRD patterns were recorded using a XD-3ACu Ka X-ray diffractometer.

3.2.1. UV absorption characteristicsAfter the suspension liquid of ZnS ages for 48 h, the sediment was dispersed in isopropanol for UV

absorption test. In our experiment, pure isopropanol was used as baseline calibration before themeasurement starts. The absorption spectrum of zinc sulfide from 200 to 400 nm is shown in Fig. 8. Itcan be seen that the strongest absorption peak of the ZnS prepared in the RPBR appears at 217 nm,which is fairly blue-shifted from the absorption edge of the bulk ZnS (345 nm) [16]. The ZnS has agood absorption for light in the wavelength range of 200–330 nm.

Fig. 6. Particle size changed with rotating speed of RPBR.

Fig. 7. Conversion changed with rotating speed of RPBR.


3.2.2. XRD analysisAfter the suspension liquid of ZnS ages for 48 h, the supernatant was removed. The sediment was

filtered, and washed by water, and then dried at the temperature of 105 8C. By sieving through 200mesh of sieve, the powder was obtained. The crystal of the product was analyzed using XRD, and theXRD patterns are shown in Fig. 9. The sample prepared in the RPBR and commercial sample werecompared. Their first strongest peak, intensity and half band width are listed in Table 1.

Fig. 8. The absorption spectrum of zinc sulfide.

Fig. 9. XRD patterns of all samples: (1) ZnS bought on the market; (2) ZnS made in RPBR.

Table 1

Eigenvalue of XRD patterns of all samples

No. Item The first strongest peak The second strongest peak The third strongest peak

2y (8) I/I1 FWHM (8) 2y (8) I/I1 FWHM (8) 2y (8) I/I1 FWHM (8)

1 ZnS, commercial

sample

28.5439 100 0.5012 47.5300 51 0.52000 56.3900 31 0.62000

2 ZnS made in

the RPBR

28.5565 100 0.31310 47.5131 55 0.30430 56.3855 35 0.32490

3 PDF Card

(sphalerite)

28.557 100 47.513 51 56.287 30


Compared with PDF Card No. 050566, on the basis of eigenvalue of XRD, the samples prepared inthe RPBR is identified as sphalerite ZnS. The diffraction peak of the ZnS prepared is sharper than thatof the vacuum coating ZnS available commercially, which indicates that the crystal of the ZnS made inthe RPBR has better crystallinity.

Based on Scherrer equation of D ¼ Kl=b cos y, where D is the average size of crystal particle, K isconstant (¼0.89), y is the prague angle and b is the increment of half band peak, the average size ofcrystal particle was calculated to be 27 nm, which coincides well with the result from TEM.

4. Conclusions

Using zinc nitrate and hydrogen sulfide as raw materials, nanosized ZnS particles were preparedunder high-gravity environment in the RPBR. Because the reaction is very rapid under high-gravityenvironment, a suitable aging time is needed for the growth of the particles. In addition, the effects ofexperimental conditions such as reactant concentration, reaction temperature and rotating speed of theRPBR on the preparation of nano-ZnS particles were also investigated. We obtain an optimumcondition for the preparation of ZnS, which is the aging time of 48 h, concentration of zinc nitrate of0.1 mol/l, reaction temperature of 45 8C and rotating speed of the RPBR of 1500–1800 rotation/min.Under these optimum conditions, well-dispersed nanosized ZnS was prepared. Simultaneously, thecrystal structure, optical properties, size and morphology of the product were also characterized byXRD, UV-Vis spectrophotometer and TEM, respectively. Results indicate that the strongest absorptionpeak of the ZnS prepared in the RPBR appears at 217 nm, which is fairly blue-shifted from theabsorption edge of the bulk ZnS (345 nm). Moreover, the ZnS prepared has a good absorption for lightin the wavelength range of 200–330 nm. XRD analysis shows the ZnS prepared is in a sphalerite crystalphase. In short, the process showed good commercialization prospects, because the raw materials canbe obtained readily and the operating parameters can be controlled easily.

Acknowledgements

This work was supported by National Natural Science Foundation of China (Grant No. 20236020), KeyProjects of Science and Technology Research of the Ministry of Education of China (Grant No. Key0202), and Innovation Development Scheme (IDS) by Economic Development Board of Singapore.

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preparation and characterization of zinc sulfide nanoparticles under high-gravity environment

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