Synthesis of ZnS nanoparticles by solid-liquid chemical reaction with ZnO and Na₂S under ultrasonic

SHE Yuan-yuan(佘媛媛)^{1, 2}, YANG Juan(杨 娟)¹, QIU Ke-qiang(丘克强)¹

1. School of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China;

2. Hunan Chemical Industry Vocation and Technology Institute, Zhuzhou 412004, China

Received 6 July 2009; accepted 30 December 2009

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Abstract: A novel and simple solid-liquid chemical reaction route was proposed to synthesize ZnS nanoparticles. In the method, ZnS nanoparticles were prepared by reaction of ZnO and Na₂S in water with ultrasonic radiation at low temperature. The effects of process parameters on the properties of ZnS particles were investigated. The products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), infrared spectroscopy (IR), thermogravimetry- differential thermogravimetry (TG-DTG) and fluorescence emission spectroscopy. The results show that these particles are good crystalline zinc blende with average size of 35 nm, and possess good IR transmittance in the range of 400 to 4 000 cm^-1 and good thermal stability in oxygen.

Key words: ZnS nanoparticle; ZnO; optical property; ultrasonic

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Nanotechnology is a new kind of technology developing rapidly since 1980s. Nanoparticles attracted considerable attention in recent years because of their special properties, such as quantum size effects[1-2] and abnormal luminescence phenomenon[3-5]. As a kind of transition metal sulfides, ZnS nanoparticles show special photoelectricity properties, such as photoluminescence (PL) and electroluminescence (EL)[6-7], thus they have extensive application in lasers, sensors, infrared windows, displays, and many other fields[8-11]. ZnS particles have two kinds of structures: zinc blende structure (cubic crystal) and wurtzite structure (hexahedron). ZnS applied in luminescent materials generally has zinc blende structure[12].

There are many methods to prepare ZnS nanoparticles, which can be divided into two categories: physical method and chemical method. The chemical method mainly includes thermal decomposition method, micro-emulsion method, sol-gel method, and LB technique. However, these methods need high reaction temperature, vast use of organic solvents, high cost of equipment operation, and complex process control. In most cases, particles prepared by these methods are of poor uniformity and agglomerate easily[13-14]. XU et al[15-16] succeeded in preparing ZnS nanoparticles by reaction of Zn nanoparticles and Na₂S solution. However, the products are low purity. In this work, we present a new method to obtain ZnS nanoparticles without any stabilizer. Taking the normal ZnO and Na₂S as raw material, combining the solid-liquid chemical reaction and ultrasonic radiation, the cubic ZnS nanoparticles with good dispersion are obtained in aqueous phase under magnetic stirring.

ZnS nanoparticles were prepared in a Na₂S solution-ZnO suspension system.

ZnO(s)+S^2-(aq)+H₂O(l)=ZnS(s, sph)+2OH^-(aq)    (1)

According to the chemical reaction isotherm equation:

             (2)

where Δ_rG_m and Δ_rG_m^θ are the Gibbs free energy and standard molar Gibbs free energies of equation (1), J/mol, respectively; R is the gas constant (8.314 J/(K?mol); T is the temperature (K); and  are the concentration of OH^- and S^2- (mol/L), respectively.

If Δ_rG_m＜0, the forward reaction will take place. The sign of Δ_rG_m is basically decided by Δ_rG_m^θ when the absolute value of Δ_rG_m^θ is great. Therefore, in general case, if Δ_rG_m^θ has a great negative value, Δ_rG_m will be also negative. The values of Δ_rG_m^θ in this reaction under different temperature are shown in Table 1. It can be seen that, preparing ZnS from the reaction of ZnO and Na₂S solution is feasible. Furthermore, the solubility products of ZnO and ZnS in water at room temperature are 3×10^-18 and 1×10^-24, respectively. ZnS is more difficult to dissolve in water than that in ZnO, which is beneficial to the forward reaction.

Table 1 Gibbs free energy change under different temperature

A typical synthesis method yielding ZnS nanoparticles is described as follows. 2 g ZnO powder and 40 mL Na₂S solution (2.0 mol/L) were first mixed and radiated by ultrasonic for a period of time, then purged with argon to remove oxygen. After being magnetic stirred in 90 ℃ water bath for 2 h, the reaction mixture was quenched to room temperature immediately, and then washed with distilled water and ethanol for several times to remove unreacted Na₂S. The ZnS nanoparticles were finally obtained after vacuum-drying and grinding.

The X-ray diffraction spectrum of ZnS nanoparticles was obtained by using CuK_α1 (λ=0.154 05 nm) irradiation on an X-ray spectrometer (Rigaku D/Max-2550V). Particle size and morphology were observed by using scanning electron microscope (KYKY2800) and transmission electron microscope (JEM-1230). The thermogravimetry-differential thermogravimetry (TG-DTG) curves were obtained by thermal weight analysis equipment (TGA/SDTA 851e) in pure oxygen atmosphere. The infrared spectrum was measured by Bruker FT-IRIFS-48 spectrum equipment. A fluorescence spectrophotometer (Hitachi F-2500) was employed to collect the photoluminescence spectrum.

3.1 Product purity

Fig.1 shows the effects of different process parameters on the purity of ZnS particles. As the temperature increasing, the mass fraction of ZnS in the product gradually increases. This is expected, because the reaction is an endothermic reaction, and increasing temperature can enhance the forward reaction and further convert ZnO to ZnS. Increasing Na₂S concentration and prolonging the stirring time within a certain range will lead to a higher purity of the product, since both of them can make contributions to the complete consumption of ZnO.

Fig.1 Effects of temperature (a), Na₂S concentration (b), and  stirring time (c) on purity of ZnS particles

3.2 Product granularity

Fig.2 shows the effect of ultrasonic time on the average particle size of ZnS particles. It can be seen that, ultrasonic radiation before reaction can facilitate the formation and stabilization of nanoparticles. Because of the cavitation of ultrasonic, the particles size decreases with extending of ultrasonic radiation time. The bubbles produced by ultrasonic field are radiated by ultrasonic at the same time. The impact effect of the cracked bubbles at liquid-solid interface results in the formation of small precursor particles. So ultrasonic radiation can accelerate the formation of the crystal in the inorganic supersaturated solution, increase nucleation rate and inhibit crystal growth. Then the fine grain can be obtained. In contrast, the size of ZnS particles obtained without ultrasonic radiation can not reach nanoscale, most of them are microscale.

Fig.2 Effect of ultrasonic time on average size of ZnS particles

3.3 Product morphology

The particle size and morphology of the samples were investigated by SEM and TEM. Fig.3(a) shows the SEM image of ZnO powder. The SEM image and TEM image of ZnS nanoparticles are shown in Figs.3(b) and (c), respectively. It can be seen that the ZnS sample consists of cubic particles with a narrow size distribution, and the average size of the particles is about 35 nm, which is much smaller than that of ZnO particles. In this work, fully crystallized ZnS nanoparticles are obtained at low temperature without calcining, which have small particle size and uniform distribution. The energy consumption of the preparation process is reduced, and the oxidation and agglomeration of ZnS powder are prevented at the same time.

Fig.3 SEM images of ZnO powder (a) and ZnS particles (b), TEM image of ZnS particles (c)

3.4 XRD analysis

The XRD patterns for ZnS nanoparticles prepared with different Na₂S concentrations are shown in Fig.4. According to Fig.4(a), the diffraction peaks match the Bragg angles measured for bulk ZnS [111], [200], [220], [311], [222], [400], and [331] of the cubic crystalline with zinc blende structure. However, the diffraction peaks of ZnO [100], [002], [101], [103] and [112] are also found. It is indicated that ZnS prepared with Na₂S concentration of 1.125 mol/L is impure, mixed with a little unreacted ZnO. From Fig.4(b), ZnS prepared with Na₂S concentration of 2.0 mol/L is purer and less impurity diffraction peaks are found.

Fig.4 XRD patterns of ZnS prepared at Na₂S concentration of 1.125 mol/L (a) and 2.0 mol/L (b)

The mean particle sizes in diameter are 17.0 nm and 27.0 nm, respectively, which are determined by Scherrer formula: D_hkl=kλ/(cos βθ_hkl), where hkl is the Miller indices, D_hkl is the mean particle size of crystal perpendicular to crystallographic orientation (hkl); k is a constant; λ is the wavelength of X-ray radiation; β is the full width at half-maximum of the (hkl) crystal face diffraction peaks; θ_hkl is Bragg diffraction angle of (hkl) crystal face. It is known that decreasing particle size results in a broadening of the diffraction peaks. The broadening of peaks in Fig.4(a) is more obvious than that in Fig.4(b), so the particle size in Fig.4(a) is smaller. The peaks of Fig.4(b) are much sharper than that of Fig.4(a), indicating that the product obtained under the condition of Fig.4(b) has more fully crystalline particles.

Compared the XRD pattern in Fig.4 with the XRD patterns of ZnS prepared by other researchers[15-16] with liquid reaction, ZnS prepared in this work performed sharper diffraction peaks, indicating that it has a better crystallinity.

3.5 IR spectrum

Fig.5 shows the IR spectrum of the ZnS nano- particles. The stretch vibration adsorption of Zn—O at 420-460 cm^-1 cannot be observed. It means that ZnS is not oxidized to ZnO during the preparation. This indicates that purging inert gas into the reacting system can prevent the oxidation of ZnS nanoparticles effectively. The stretch vibration absorption of O—H at 2 900-3 600 cm^-1 also cannot be observed, which means that vacuum drying can remove the water adsorbed on the particles. There are few absorption peaks at 400-4 000 cm^-1, indicating that the prepared nano-ZnS is a nicer infrared-transmittance material.

Fig.5 IR spectrum of ZnS nanoparticles

3.6 Thermoanalysis

TG-DTG measurement on the sample prepared with Na₂S concentration of 2.5 mol/L was carried out, as shown in Fig.6. Due to the strong water absorption of ZnS[17], it adsorbs water from oxygen, which leads to the slight increase of mass at 50 ℃. There is a mass loss around 100 ℃, which can be attributed to the elimination of adsorbed water on the particle surface. When the temperature is over 100 ℃, the mass of sample is almost constant until 570 ℃, and then has a rapid mass loss of about 14% at 620 ℃. It can be thus inferred that ZnS is oxidized to ZnO at 570 ℃ in oxygen[16]. The ultimate mass loss is about 17%. It can be concluded that ZnS particles possess good thermal stability in oxygen.

Fig.6 TG-DTG curves of ZnS nanoparticles

3.7 Fluorescence emission spectrum

In order to study the optical properties of the prepared ZnS nanoparticles, fluorescence emission spectrum analysis was carried out. Fig.7 shows that the ZnS nanoparticles prepared in this work have a single emission peak at about 445 nm. The intensity of emission peak is strong and the emission band is narrow, which further indicates the ZnS nanoparticles have a good crystallinity.

Fig.7 Fluorescence emission spectrum of ZnS

1) ZnS nanoparticles are synthesized by a simple solid-liquid chemical reaction without any surfactant. The preparation process has advantages of simple technology, high yield, cheap raw material and short preparation cycle.

2) The prepared ZnS nanoparticles are good crystalline cubic zinc blende with a uniform distribution and an average size of 35 nm. Few impurity peaks are found in the XRD patterns, indicating that the products are high purity. The prepared nanoparticles are nice infrared-transmittance materials with good crystallinity, infrared transparency and thermal stability.

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Corresponding author: QIU Ke-qiang; Tel: +86-731-88836994; Fax: +86-731-88879616; E-mail: qiuwhs@sohu.com

(Edited by LIU Hua-sen)