Evolution of ZnS Nanoparticles via Facile CTAB Aqueous Micellar Solution Route: A Study on Controlling Parameters
© to the authors 2008
Received: 11 August 2008
Accepted: 17 October 2008
Published: 6 November 2008
Synthesis of semiconductor nanoparticles with new photophysical properties is an area of special interest. Here, we report synthesis of ZnS nanoparticles in aqueous micellar solution of Cetyltrimethylammonium bromide (CTAB). The size of ZnS nanodispersions in aqueous micellar solution has been calculated using UV-vis spectroscopy, XRD, SAXS, and TEM measurements. The nanoparticles are found to be polydispersed in the size range 6–15 nm. Surface passivation by surfactant molecules has been studied using FTIR and fluorescence spectroscopy. The nanoparticles have been better stabilized using CTAB concentration above 1 mM. Furthermore, room temperature absorption and fluorescence emission of powdered ZnS nanoparticles after redispersion in water have also been investigated and compared with that in aqueous micellar solution. Time-dependent absorption behavior reveals that the formation of ZnS nanoparticles depends on CTAB concentration and was complete within 25 min.
There has been great interest over the years to improve the fundamental understanding of CTAB aqueous micellar system. However, some aspects particularly the factors controlling synthesis of nanomaterial in aqueous solution of surfactant are still not very well understood. Further efforts are being made to control the shape and size of nanoparticles using surfactant aggregates. Unfortunately, the use of surfactant monomers/assemblies to control the shape and size of nanoparticles remains an extremely difficult task, since the surfactant adsorption and aggregation processes itself is affected by many kinetic and thermodynamic factors. These factors will have an obvious effect on nanoparticles synthesis in aqueous micellar media. Increasingly, chemists are contributing to understand the synthesis, mechanism, and novel properties of semiconductor nanoparticles using various surfactants. Of the various type of nanocrystals, semiconducting metal chalcogenide nanocrystals have been most intensive studied because of their interesting effects such as size quantization [1, 2], non-linear optical behavior , photoluminescence , and so on. The increase in band gap with decrease in particles size is the most identified aspect of quantum confinement in semiconductors. ZnS is a wide band gap semiconductor with band gap energy (E g) of 3.68 eV. It has been widely used in many optoelectronic devices such as blue-light-emitting diode, solar cells, and field emission devices [5–7]. Their synthesis has been achieved via various routes, including hydrothermal synthesis, aqueous micelles, reverse micelles, sol–gel process, and spray pyrolysis [8–12].
Considerable experimental work has been performed in the past in order to synthesize and understand the properties of ZnS nanoparticles with and without using surfactants [13–15]. Cao et al. synthesized ZnS nanotubes taking CS2 as sulfide ions source at high temperature and using Triton X-100 as micellar template. Wu et al. obtained winding ZnS nanowires from reverse micelle solution. Mitra et al. prepared ZnS nanoparticles in aqueous solution of anionic surfactant, sodium dodecylsulfate (SDS), and studied the effect of surfactant only at concentrations above critical micellar concentration of SDS. To synthesize nanoparticles with well-defined shapes and sizes, detailed understanding of stabilization mechanism and controlling parameters is required. Furthermore, one of the typical features of nanoparticles is their spontaneous self-aggregation into functional structures driven by the energetics of the system, which are known as self-aggregated nanostructures. Though in solution the nanoparticles may be well separated, during separation process, some of the particles may get agglomerated. Thus, the effectiveness of any synthetic method can be defined in terms of the percentage of particles obtained within the required size range and extent of self-agglomeration during separation process. There are only few reports [18, 19] on systematic investigations of ZnS nanoparticles using CTAB aqueous micellar media that provides detailed understanding of stabilization mechanism. It is well established in literature  that the rate of adsorption of cationic surfactants is very fast and the final amount adsorbed is higher than anionic and non-ionic surfactants. Therefore, if adsorption is thought to be the criteria for the stabilization of nanoparticles, then size, shape, and other properties of the nanoparticles in cationic surfactant like CTAB must differ from those in anionic and non-ionic surfactants.
Keeping the above points in view, we report the results related to various parameters controlling the synthesis and stabilization of ZnS nanoparticles in aqueous solution of CTAB. In addition to other characterization techniques, time-dependent absorption behavior has been used to investigate the effect of surfactant on nanoparticles growth process.
Synthesis of CTAB-Capped ZnS Nanoparticles
Cetyltrimethylammonium bromide (CTAB, sigma, 99%), Zn(OAc)2 · 2H2O (CDH, 99.5%), Na2S · xH2O (CDH, 55–58% assay) all analytical grade have been used as received. Aqueous solution of CTAB, Zn(OAc)2 · 2H2O (0.025 M), and Na2S · xH2O (0.025 M) was prepared in double distilled water. The aqueous solution of CTAB was stable for months together except at temperature below 288.15 K. ZnS nanoparticles were prepared using simple precipitation method described by Han et al. with some modifications. In the typical procedure, the CTAB micellar solution containing Na2S was added dropwise to another containing Zn(OAc)2 with constant stirring in a thermostated vessel maintained at 298.15 K. The solution was then allowed to stand for 30 min at the same temperature. The concentrations of both the salts in aqueous micellar solution were varied between 0.1 and 0.7 mM. The nanoparticles in aqueous micellar media were then subjected to UV-vis, SAXS, fluorescence, and TEM measurements. The ZnS nanoparticles were separated from solution by slow evaporation of solvent at 50–55 °C. The particles were isolated, washed with water and ethanol, and then again dried at 50–55 °C. The dried powder was collected and subjected to XRD, SEM, and FTIR measurements. The material was redispersed in water to again perform TEM, fluorescence, and absorption measurements.
Characterization of Nanoparticles
The ZnS nanoparticles were characterized using Hitachi (H-7500) Transmission electron microscope (TEM) 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 the dried sample were taken using Jeol (JSM-6100) scanning microscope operating at 25 kV. FTIR spectra of dried ZnS nanoparticles were recorded with Perkin Elmer RX-1 spectrophotometer. Powder X-ray diffraction (XRD) patterns were observed on STOE Transmission diffractometer (STADI-P) equipped with Cu-kα radiation (λ = 1.5418 A°). UV-vis spectra of the nanodispersions were recorded in Jasco-530 spectrophotometer with matched pair of quartz cell of 1 cm path length. Fluorescence spectra were recorded on Varian fluorescence spectrophotometer. pH measurements were carried out at 298.15 K with Cyberscan-510 pH meter. UV-irradiation of samples has been performed using Ultraviolet Fluorescence Cabinet (PT-32/24; Popular India; intense lines at 254 and 365 nm). Optical measurements and other studies were all carried out at room temperature under ambient conditions. SAXS measurements were done on the beamline ID02 of the European Synchrotron Radiation Facility (ESRF, Grenoble, France). The SAXS intensity was recorded on a 2D-CCD detector, corrected for background and scattering of the empty capillary, and converted into absolute units by standard procedures using a standard of known scattering intensity.
Results and Discussion
Formation and Optical Properties of ZnS Nanoparticles
Theoretically, the ratio [Zn(OAc)2:[Na2S] would be 1:1. But actually [S2− < [Na2S], because aqueous solution of Na2S contained both aqueous H2S and HS− as well as other sulfur oxyions such as thiosulfate and sulfite, originating either as impurities in solid Na2S or from rapid oxidation of HS− by O2. Thus, some preliminary experiments of ZnS nanoparticles formation in aqueous solution of CTAB were undertaken to develop an understanding of the [Zn(OAc)2:[Na2S] ratio, which leads to the formation of maximum ZnS nanoparticles.
Optical band gap (E g) and nanoparticle diameter (d abs) as calculated from tauc plots and Wang equation
Photoluminescence (PL) Studies
Figure 2b compares the room temperature PL spectra of the ZnS nanocrystals in aqueous micellar solution and that of ZnS nanopowder redispersed in water. In both the measurements excitation wavelength was 320 nm. The ZnS nanocrystals in aqueous micellar solution of CTAB exhibit three emissions peaking at 383, 424 and 462 nm, and redispersed ZnS shows two intense emissions at 424 and 462 nm and one weak emission at 380 nm. The interesting point is that the intensity shows reciprocal trends in two samples, i.e., the emissions that are strong in one become weak in the other and vice versa.
This type of behavior can be attributed to change in shape and size of nanocrystals during separation and drying process as the luminescence spectra show size- and shape-dependent quantum confinement effects. In literature, the emissions at ~383 and at ~423 nm have been assigned to shallow-trap and deep-trap emissions or defect-related emission of ZnS, respectively [28, 29]. Han et al. have also observed similar type of defect-related emissions near 430 nm for CTAB passivated ZnS. The change in intensity of these emissions can be explained in terms of surface passivation by sulfide ions and surfactant molecules and unpassivation during separation and drying process . The nanocrystals in aqueous micellar solution are surface passivated by excess sulfide ions and surfactant monomers and show weak deep-trap (intense shallow-trap) emission, whereas due to removal of passivation after redispersion the defect-related emission (423 nm) became more intense due to defects in nanocrystals. The peak at ~462 nm has been assigned to the presence of sulfur vacancies in the lattice . ZnS nanocrystals contain excess of sulfur in aqueous micellar solution, and thus show weak emission due to sulfur vacancies, but the emission became intense when excess of sulfur has been removed from the redispersed sample.
The agglomeration behavior of nanoparticles during separation and drying process has also been studied by calculating the size of nanoparticles by performing UV-vis, XRD, and SEM measurements on dried samples. The inset in Fig. 2b shows the absorption spectrum of powdered ZnS nanocrystals redispersed in water. A minor absorption shoulder peaking at 313 nm (3.96 eV) is observed. The particle size corresponding to this peak was calculated to be 6.7 nm. However, the particles seem to be much agglomerated in the powder form as evident from SEM micrographs (discussed in subsequent section). From these observations, we can infer that during drying process particles get agglomerated to some extent, but the particles have good tendency of redispersion in water.
TEM and SEM Analysis
Figure 3c, d shows the typical TEM images of the product redispersed in water and powdered sample, respectively. Nanoparticle aggregates are clearly visible in TEM micrograph. The magnified view of such an aggregate containing 8–10 particles is shown in Fig. 3d. By randomly measuring over 40 such clusters, we confirmed the size to be 6–15 nm with a few particles having a size more than 15 nm but less than 60 nm. However, most of the particles have the size 4–10 nm. The spherical morphology of synthesized particles is clearly displayed in the inset of Fig. 3d, which shows fully grown single particle. A typical low magnification SEM image of the powdered sample is shown in Fig. 3e revealing some spherical nanoparticles with most of the particles in the form of agglomerates of irregular shape. The corresponding high magnification SEM images in Fig. 3f display that nanoparticles are attached to one another. The shape and size of ZnS nanoparticles have been found to be different from those prepared in other surfactants. Cao et al. reported ZnS nanorods in Triton X-100 at higher temperature whereas Mitra et al. synthesized triangular-shaped nanoparticles in SDS aqueous micellar solution.
Small-Angle X-ray Scattering (SAXS)
Lower (0.3 < q < 0.45 nm−1) and upper (q < 0.2 nm−1) limit for the particle radiusR and the particle radius as derived from the mean particle volume according to Eq. 6
R(q < 0.2 nm−1)
R(0.3 < q < 0.45 nm−1)
XRD and FTIR Studies
where n is the agglomeration number, N a is Avogadro’s number, V m is the molar volume of ZnS in cm3 mol−1, and r is nanoparticle radius. We calculated the agglomeration number to be 2597 for r = 2.9 nm. The number of ZnS units contained in a nanoparticles was further confirmed by using another simple method taking into account the lattice parameter, a, calculated above. (The equations for calculating the particle agglomeration number using both the methods are given in Appendix A.)
Adsorption of CTAB on ZnS nanoparticles was examined by recording the FTIR spectra in the range 4,000–400 cm−1. Figure 5b depicts the FTIR spectra of CTAB and CTAB-capped ZnS nanoparticles. From Fig. 5b, it is to be noted that the symmetric and asymmetric –CH2 stretching vibrations of pure CTAB lie at 2,914 and 2,846 cm−1 and remained almost same in the presence of ZnS nanoparticles within the experimental errors. The peaks at 1,550 and 1,474 cm−1 for pure CTAB are attributed to –C–H scissoring vibrations of –N–CH3 moiety , which are shifted to 1,595 cm−1 in the presence of ZnS nanoparticles. Also the peaks at 1,252 and 1,209 cm−1 due to –C–N stretching are suppressed and significantly shifted to 1,212 and 1,067 cm−1 in the presence of ZnS NPs. Therefore, from FTIR results, it is clear that the peaks due to CTAB head group region are shifted without any significant shift in hydrocarbon tail region. These results confirm the stabilization of ZnS nanoparticles by adsorption of CTA+ through head group region as hypothesized on the basis of pH studies (discussed later in this paper).
Role of CTAB
Since CTAB is a cationic surfactant, Zn2+ions would not be adsorbed on the micelles. But S2−and HS−ions generated by the ionization of Na2S would interact with CTA+. Also, the ratio of [Zn(OAc)2]:[Na2S] during the synthesis of ZnS nanoparticles was maintained on 1:2; hence it is suggested that ZnS nanoparticles are capped by CTA+with excess HS−ions adsorbed on the surface of surfactant aggregates.
Effect of pH on Precipitation and Stabilization of ZnS Nanoparticles
Thus, in basic medium, more S2− ions are available to combine with Zn2+ forming more ZnS nanoparticles, whereas in acidic medium S2− ions are being converted into HS− ions. Also it was noted that particles get agglomerated at low and very high pH due to lack of effective capping by surfactant molecules. The ZnS particles were negatively charged in the pH range of 5.3 < pH < 9.3, and negatively charged species such as Br− or HS− face an electrostatic barrier to surface adsorption . Thus, it is hypothesized that the ZnS nanoparticles are stabilized by the adsorption of CTA+ through ammonium headgroup due to electrostatic interactions, forming surfactant bilayer on the surface of nanoparticles. The counterions (Br− and HS−) are present at the surface of bilayer thus generating excess negative charge again. This type of effective stabilization is not present at low and very high pH and the particles gets agglomerated. Formation of CTAB capped ZnS nanoparticles were also confirmed by FTIR studies described earlier.
Nanoparticles Growth in Presence of Surfactant
The time-dependent absorption behavior of ZnS nanoparticles was also investigated by measuring the UV-absorption at 294 nm as a function of time at different CTAB concentrations with constant Zn(OAc)2 = 0.5 mM and Na2S = 1.0 mM. The mixing time in these studies was also 40–45 s. Therefore, time ‘zero’ was on the order of 40–45 s after mixing and the reaction was monitored for 80 min. It can be depicted from Fig. 7b that the absorbance first increases rapidly within the mixing time (40–45 s) and then increases steadily to reach the maximum value. After reaching the maximum value, absorbance decreases with a very small plateau region of constant absorbance.
The absence of the plateau region of constant absorbance in all CTAB concentrations reveals that the process of decay has started before the growth was completed. The effect of UV-radiations on nanoparticles was found to be least at high CTAB concentration, i.e., 5 mM. This type of behavior might be due to passivation of ZnS surface by surfactant molecules and prevent the direct impact of UV-light.
The ZnS nanoparticles have been prepared in aqueous micellar solution of CTAB. On the basis of various studies reported in the paper, the nanoparticles are found to effectively cap adsorption of CTA+through head group. The adsorption process is pH dependent, as the particles are more stable over a particular pH range. TEM and SEM images show the spherical morphology of the nanoparticles, but due to agglomeration there may be some change in shape, which is confirmed by SAXS experiments in the liquid state. The dried samples have also been characterized using TEM, absorbance, and fluorescence emission and compared with those in aqueous micellar solution. The results reveal that although particles show agglomeration in powdered form, they show good tendency for redispersion in water. Fluorescence studies reveal some crystal defects in the nanoparticles during separation drying process. The time-dependent adsorption behavior reveals that stabilization of nanoparticles by CTAB follows different mechanisms at different CTAB concentrations. The exact mechanism is still not very clear and further studies are to be carried out in this context. All these findings seem to be very useful to define the stability of ZnS nanoparticles during synthesis in aqueous micellar media.
Calculation of Agglomeration Number
where r is the radius in nanometers.
Sanjay Kumar is thankful to CSIR, India, for fellowship. S. K. Mehta and Michael Gradzielski are grateful to DST and DAAD for the award of Project Based Personal Exchange Programme (PPP)-2008. We would like to thank T. Narayanan and P. Panine from ESRF and P. Heunemann from TU Berlin for help with the SAXS measurements.
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