Effect of Cationic Surfactant Head Groups on Synthesis, Growth and Agglomeration Behavior of ZnS Nanoparticles
© to the authors 2009
Received: 24 December 2008
Accepted: 15 June 2009
Published: 1 July 2009
Colloidal nanodispersions of ZnS have been prepared using aqueous micellar solution of two cationic surfactants of trimethylammonium/pyridinium series with different head groups i.e., cetyltrimethylammonium chloride (CTAC) and cetyltrimethylpyridinium chloride (CPyC). The role of these surfactants in controlling size, agglomeration behavior and photophysical properties of ZnS nanoparticles has been discussed. UV–visible spectroscopy has been carried out for determination of optical band gap and size of ZnS nanoparticles. Transmission electron microscopy and dynamic light scattering were used to measure sizes and size distribution of ZnS nanoparticles. Powder X-ray analysis (Powder XRD) reveals the cubic structure of nanocrystallite in powdered sample. The photoluminescence emission band exhibits red shift for ZnS nanoparticles in CTAC compared to those in CPyC. The aggregation behavior in two surfactants has been compared using turbidity measurements after redispersing the nanoparticles in water. In situ evolution and growth of ZnS nanoparticles in two different surfactants have been compared through time-dependent absorption behavior and UV irradiation studies. Electrical conductivity measurements reveal that CPyC micelles better stabilize the nanoparticles than that of CTAC.
The synthesis of ultrafine semiconducting particles is of great technological and scientific interest due to their superior physical and optical properties. Zinc sulfide (ZnS) is an important wide band gap (3.60 eV) semiconductor and used as a key material for large range of applications [1–3]. Over the years, attempts have been made to prepare, stabilize and isolate homogeneously dispersed ZnS nanoparticles with and without capping agents [4–7]. When these clean nanoparticles aggregate, they lose their nanoscale sizes and corresponding properties. Therefore, in addition to tune particle size, a low degree of agglomeration and monodispered size distribution are desirable to enable homogeneous arrangement of particles. Due to partially satisfactory results, available methods still represents a major challenge to date and ultimate aim of the current research in material science is to understand the mechanisms that determine the crystal habitat and shape of the crystal. In last few years, extensive structural, kinetic and thermodynamic studies have been performed to explore the fundamental understanding of surfactant–water system including the effect of additives on micellization [8–10]. However, still there are conflicting opinions on some aspects particularly, the studies regarding factors controlling the synthesis and stabilization of nanoparticles in aqueous surfactant solutions. Therefore, it is quite difficult to scale up a general method for the nanoparticles synthesis using surfactants, because numerous parameters with different influences enter in to consideration, while studying a particular system.
One interesting aspect, which should be mainly considered, is directly related to particle size control by the adsorption of surfactant onto the particles surface. Among several methods to prevent self-aggregation of nanoparticles, coating with surfactants, where one end of the surfactant chain is anchored to particle surface and other end is free, is simple and effective method to first give one dimensionally ordered self-assembly and then higher dimensional close-packed superlattice . The surfactant coating on nanoparticles changes their aggregation behavior due to changed interparticle potential. Therefore, different types of surfactants, depending upon their molecular structures, may tune the interparticle interactions to different extent and hence have different tendency to prevent the nanoparticles aggregation. Apart from the synthesis purpose, surfactants have been used in association with nanoparticles for variety of studies [12, 13]. Zaman et al.  has investigated the interparticle forces and stability of silica dispersions in C12TAB through turbidity and viscosity measurements. Keeping in view the importance of surfactant–nanoparticles system, it would be very interesting to know whether there is any influence of surfactant structure on size, shape, stability and other properties of nanoparticles. A comparative study of a particular system in different surfactants can provide a better insight into the nanoparticles stability and properties. Naskar et al.  compared effect of two nonionic surfactant stabilized emulsions on ZnS nanoparticles size. Shao et al.  studied the role of oleic acid and TOP on growth and agglomeration behavior of cobalt nanoparticles synthesized via thermal decomposition. However, there is hardly any report on the comparative studies of ZnS nanoparticles in cationic surfactants till date.
Materials and Methods
For the synthesis of ZnS nanoparticles, Zn(OAc)2·2H2O (99.5%), Na2S·xH2O (55–58% assay), all were of analytical grade obtained from central drug house (CDH). The surfactants, CTAC (99%) and CPyC (99%), were obtained from Fluka and Himedia, respectively. All reagents were used as received, without further purification. The solvents acetone and ethanol were AR grade products.
ZnS Nanoparticle Synthesis
Two micellar solutions of CTAC (3 mM), one containing Zn(OAc)2·2H2O (0.025 M) and another containing Na2S·xH2O (0.025 M), were prepared in double-distilled water. The synthesis of ZnS nanoparticles was performed by two-step procedure. The first step involves the generation of the S2−-surfactant complex by adding aqueous sodium sulfide (0.025 M) to aqueous surfactant solutions. In the second step, dropwise addition of aqueous micellar solution containing Zn(OAc)2(0.025 M) into the above solution with constant stirring at ambient temperature leads to the formation of ZnS nanoparticles. The homogeneous solution was then allowed to stand for 30 min at room temperature. The dispersions were found to be stable for months together. The nanoparticles were separated by slow evaporation of solvent at 50–60 °C. The collected solid product was washed with double-distilled water and ethanol and then vacuum dried for 48 h. We also tried ultracentrifugation, but nanoparticles got badly agglomerated. Similar procedure was followed for the synthesis of ZnS nanoparticles in CPyC.
UV–vis Absorption Spectroscopy
Optical spectra of the nanodispersions were taken with a JASCO-530 V spectrophotometer in quartz cuvette of 1 cm path length. For time-dependent absorption measurements, two solutions were mixed and immediately transferred to quartz cuvette. The mixing time was about 40–45 s before starting the absorbance measurement. The measurements were then taken at the rate of 12 measurements per minute. UV irradiation experiments were carried out in Popular India UV cabinet.
Transmission electron microscopy (TEM) micrographs were taken using Hitachi (H-7500) transmission electron microscope operating at 80 kV. Samples for TEM studies were prepared by placing a drop of nanodispersion on a carbon-coated Cu grid, and the solvent was evaporated at room temperature. SEM images of powdered sample were taken using JEOL (JSM-6100) scanning microscope.
Dynamic Light Scattering
The dynamic light scattering (DLS) measurements were taken on ALV-5000 with Nd:YAG laser with a wavelength of 532 nm. Multiple tau digital correlation was measured at the minimum sampling of 6.25 ns using a dual auto correlation mode on an ALV-5000 correlator board. All measurements were taken at scattering angle of 90° for different suspensions. A sample cell was set in the toluene bath for index matching with the quartz. The temperature was maintained at 25 °C in the toluene bath.
X-Ray Diffraction Studies
Powder XRD studies were carried out using Panalytical, D/Max-2500 X-Ray Diffractometer equipped with Cu-kα radiation (λ = 1.5418 Å) employing a scanning rate of 0.02° s−1. Si was used as standard to determine the instrumental broadening, and the (111) reflection was analyzed. The ∆2θ for the silicon peak was about 0.06 (θ), and a simple instrumental correction was carried out by subtracting this value from the ∆2θ values corresponding to the diffraction peaks obtained for our samples.
FTIR spectra of dried ZnS nanoparticles were recorded with Perkin Elmer RX-1 spectrophotometer in frequency range of 4,000–900 cm−1. Small amount of sample was mixed with 2–3 drops of CCl4to form a thick paste. The paste was then applied on NaCl plates to record the spectra.
The PL spectra were recorded on Varian fluorescence spectrophotometer. The excitation wavelength of 320 nm was used, and PL emission was recorded in 330–560 nm range.
Turbidity measurements of redispersed ZnS nanopowder were taken in a digital turbidity meter (Decibel Instruments) with an accuracy of ±3% of full-scale deflection. Powdered ZnS nanoparticles (0.04 g) were dispersed in 35 mL water and sonicated for 30 min, then kept undisturbed in glass cuvette in the cuvette holder of turbidity meter. Turbidity of solution (in NTU) was noted after regular intervals.
The specific conductivity measurements of aqueous surfactant solutions in the presence of ZnS nanoparticles were measured using PICO digital conductivity meter operating at 50 Hz from Lab India instruments with an absolute accuracy of ±3%. Platinised platinum electrode was inserted in a double-walled vessel containing the solution in which the thermostated water was circulated. The conductivity cell was calibrated with standard KCl solutions, and the obtained cell constant was 1.02 cm−1.
Results and Discussion
Formation of ZnS Nanoparticles and Optical Characterization
Electron Microscopy and DLS
X-Ray Diffraction Studies
Average crystallite sizes and amount of strain of ZnS nanoparticles calculated on the basis of powder XRD analysis
11.0 ± 0.2
13.4 ± 0.3
4.6 ± 0.6
10.8 ± 0.2
13.1 ± 0.3
4.3 ± 0.6
The turbidity results therefore reveal that ZnS nanoparticles prepared in aqueous micellar solution of CPyC do not form permanent aggregates during separation and drying process and have good redispersion tendency when compared to those prepared in aqueous micellar solution of CTAC. It can be thought that the adsorbed surfactant molecules remained intercalated between the particles during separation and drying process, preventing their permanent fusion to form bigger particles and get redispersed when dissolved in water. The presence of surfactant molecules in powdered nanoparticles has also been evidenced from FTIR studies.
Assignment of FTIR peaks of CTAC and CPyC capped ZnS nanoparticles
Peak position (cm−1)
CTAC + ZnS NPs
CPyC + ZnS NPs
The synthesized ZnS nanoparticles are found to have cubic crystal lattice, and Schottky defects are dominant in cubic ZnS . Therefore, deep traps in cubic ZnS involve Zn2+ and S2− vacancies. The broad, low intense, deep trap emission band at ~424 nm reveals few defects in the synthesized nanoparticles in both the surfactants . Furthermore, the narrow emission band indicates the formation of nanoparticles with narrow size distribution . The PL intensity of ZnS nanoparticles prepared in CPyC was found to be less, because of the interactions between pyridine and surface point defects of ZnS nanoparticles. The CPyC is effective in quenching the luminescence  due to the ability of N-atom in pyridinium cation to seize the electrons from the surface states of nanoparticles making the electron transfer easy. These results indicate that the photophysical properties of ZnS nanoparticles depend up on the size and surface passivation, which might help to further understand the physical mechanism of ZnS nanoparticles that give rise to PL properties.
Kinetics of Particle Formation
The growth of ZnS nanoparticles in CTAC is faster than UV-induced decay, and resultant effect seems to increase in absorbance only. On the other hand, in the presence of CPyC, nanoparticles growth is slow and decreases with time. At one stage, the nanoparticles growth becomes so slow that UV-induced decay overcomes the growth, and overall effect remains decay only.
The UV light can degrade the nanosized particles much faster due to their large surface area . The fast growth in the case of CTAC leads to larger size particles (small surface area) and, therefore, UV light-induced decay is slow when compared to growth. On the other hand, in CPyC, the surfactant molecules stabilize the particles at small size (large surface area), and hence the particles are more prone to decay due to their large exposed surface area to UV light. Even some of the small particles disappeared leading to decrease in absorbance. In addition, the head group area of CPyC is more when compared to that of CTAC [37, 38]. Therefore, the particles could not grow and got stabilized at smaller size due to adsorption of large head group of CPyC.
Furthermore, the effect of UV radiation of two different wavelengths (254 and 365 nm) on ZnS nanoparticles has also been investigated. The plots are shown in Fig. S2 (supplementary material). The results depict that short wavelength or high energy radiations degrade the nanoparticle to a larger extent than longer wavelength (low energy) radiations irrespective of the nature of surfactants.
Aggregation Behavior of Surfactants in Presence of ZnS Nanoparticles
The aggregation behavior of both the surfactants in the presence of respective nanoparticles has also been studied. When dissolved in water at a concentration below critical micellar concentration (cmc), the surfactant behaves as a strong electrolyte, whereas above the cmc, the monomers form aggregates called micelles. The process of aggregation is affected due to temperature, solvents and presence of any other external entity.
Figure 11a depicts that ZnS nanoparticles (synthesized in CTAC) are better dispersed in aqueous solution of CTAC until cmc. After that nanoparticles settled down, and CTAC micelles behave like that of pure CTAC. It indicates that soon after the formation CTAC micelles, the ZnS nanoparticles agglomerates and settles down. However, in aqueous solution of CPyC in the presence of nanoparticles, the nature of conductivity curves remains same even after cmc because of the fact that ZnS nanoparticle (synthesized in CPyC) remained suspended even after formation of micelles. This is due to more electrostatic attraction provided by larger head group size of CPyC for the stabilization of nanoparticles. The conductivity studies, thus, reveal that CPyC micelles solubilized the nanoparticles better when compared to CTAC micelles.
The ZnS nanoparticles have been prepared in aqueous micellar solution of two cationic surfactants viz. CTAC and CPyC having different hydrophilic head groups. The studies reveal that the stabilization and agglomeration of ZnS nanoparticles in aqueous micellar media depends up on surfactant head group. Based on UV–vis and turbidity experiments, CPyC has been found to provide better stabilization when compared to that by CTAC. Dependence of photoluminescence emission on the size and surface passivation of nanoparticles has also established. The studies on dependence of photophysical properties of ZnS nanoparticles on surfactant head group will be helpful in defining its priorities for optical applications.
Sanjay Kumar is thankful to CSIR, Government of India for Junior Research Fellowship. Financial assistance from DST is gratefully acknowledged.
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