The strategy and technique exploited in the synthesis of nanostructure materials have an explicit effect on the nucleation, growth, and properties of product materials. Nanoparticles of zinc sulfide (ZnS) have been synthesized by new infrared radiation (IR) assisted and Stokes’ law based controlled bottom-up approach without using any capping agent and stirring. IR has been used for heating the reaction surface designed in accordance with the well-known Stokes law for a free body falling in a quiescent fluid for the synthesis of ZnS nanoparticles. The desired concentration of aqueous solutions of zinc nitrate (Zn(NO3)2·4H2O) and thioacetamide (CH3CSNH2) was reacted in a controlled manner by IR radiation heating at the reaction area (top layer of reactants solution) of the solution which results in the formation of ZnS nanoparticles at ambient conditions following Stokes’ law for a free body falling in a quiescent fluid. The phase, crystal structure, and particle size of as-synthesized nanoparticles were studied by X-ray diffraction (XRD). The optical properties of as-synthesized ZnS nanoparticles were studied by means of optical absorption spectroscopic measurements. The optical energy band gap and the nature of transition have been studied using the well-known Tauc relation with the help of absorption spectra of as-synthesized ZnS nanoparticles.
The discovery and advancement in the new experimental techniques for the synthesis of functional materials at nanodimensions provide opportunities for the development of new and exciting research in nanostructure materials. The behavior of materials at the nanoscale can be exceptionally different from those at a larger scale and result in the deviation of device performance. As the dimensions of a material are reduced from a large size, initially the properties remain the same, then small changes occur, and finally when the sizes are below ~100 nm, remarkable changes in properties can take place. Therefore, there are endless possibilities for improved performance of devices, structures, and materials that can be realized by understanding the behavior and phenomena at nanoscale. Nanoparticle size confers unique size, shape, and orientation dependent properties such as surface plasmon resonance (SPR) in some metal particles, quantum confinement in semiconductor quantum dots, and superparamagnetism in magnetic materials. There are many examples of macroscopic manifestation of materials properties in semiconductors due to nanosized effects: more surface area (surface energy) to speed up almost all physical and manufacturing process [
The control of exact size, shape, and orientation is still a great challenge in the synthesis of nanostructure materials using a bottom-up approach (chemical methods). The bottom-up approach is typically better and more reliable for mass production due to its ease, cost effectiveness, and purity. Among the “bottom-up approaches,” coprecipitation is a most common and widely used method for preparation of nanoparticles. It is a simple, cost-effective and high yielding method. Precipitation reaction occurs when a cation and an anion of two aqueous solutions react in accordance to their solubility rules to form an ionic insoluble solid known as a precipitate in the solution where the reaction is taking place. It is possible by using the coprecipitation method to change the shape, size, and composition of the synthesized product with high reproducibility. The reaction conditions such as solution molarity and pH, rate and manner of addition of the precipitator agent, and stirring time and temperature play a very important role to achieve the desired size, shape, and orientation of the final product. Recently, we have devised an entirely new IR assisted controlled method for the synthesis of CdS nanoparticles by means of a kinetic approach using the well-known Stokes’ law for a free body falling in a quiescent fluid [
Zinc sulfide (ZnS) is a well-known II-VI semiconductor. It has attracted much research interest due to its potential applications in many solid state devices including light-emitting diodes (LEDs), electroluminescence, flat panel displays, infrared windows, sensors, lasers, and biodevices. ZnS has two commonly available allotropes: one with a cubic (zinc blende) structure and another one with a hexagonal (wurtzite) structure. The cubic form is the stable low-temperature phase, while hexagonal form is the high-temperature polymorph which forms at around 1296 K [
A survey reveals that a large number of techniques for preparing ZnS nanoparticles have been applied by different workers such as solid state reaction [
ZnS nanoparticles were prepared through an experiment designed in accordance with Stokes’ law for a free body falling in a quiescent fluid using IR radiation lamp as a heating source. The experiment mainly consists of a long beaker (~30 cm) for free falling of particles, IR radiation lamp (commercial) for heating the upper surface of the solution, and burette to add the anion (sulfur ions) solution to the cation (zinc ions) solution in a controlled manner. All the chemicals are of analytic grade and used without further purification. The entire process is carried out in distilled water for its inherent benefit of being straightforward and environmentally friendly. Nanoparticles of ZnS were made by controlled reaction through a chemical precipitation method using an aqueous solution of zinc nitrate (Zn(NO3)2·4H2O) and thioacetamide (CH3CSNH2). The reaction was controlled with the help of a drop-by-drop addition of the aqueous solution of thioacetamide (CH3CSNH2) into the aqueous solution of zinc nitrate (Zn(NO3)2·4H2O) in the presence of continuous IR lamp heating on the top as shown in schematic diagram Figure
Schematic illustration of controlled IR assisted chemical precipitation method based on Stokes’ law for a free body falling in a quiescent and viscous fluid as used for synthesis of ZnS nanoparticles.
The phase, crystallinity, and particle size of the as-prepared ZnS nanoparticles were characterized by X-ray powder diffraction (XRD) using a Bruker, D8 Advance X-ray diffractometer with CuK
The fact that X-rays have a wavelength of the order of angstroms, which is suitable for interatomic distances in solids, makes this technique an excellent instrument to investigate the phase and crystalline structure of the materials. Figure
Lattice constants as-prepared ZnS nanocrystals.
S. number | Plane ( |
|
|
|
---|---|---|---|---|
( |
(111) | 3.08 | 5.33 | 5.33 |
( |
(220) | 1.88 | 5.32 | |
( |
(311) | 1.61 | 5.33 | |
( |
(400) | 1.33 | 5.32 | |
( |
(331) | 1.23 | 5.36 |
XRD pattern of as-prepared ZnS nanoparticles.
It is well-known that XRD line broadening is influenced by the crystallite size and the internal strain present in the sample. The crystallite size (
Williamson-Hall plot (
The crystallite size (
Semiconductors behave very differently, in comparison with bulk, when their crystal sizes are reduced down to the order of a few nanometers. One of the most significant characteristic in these materials is the variation of the band gap when the sizes of nanocrystal decrease below the Bohr exciton radius (
Absorption spectra of as-prepared ZnS nanocrystals.
The absorption spectra show that the optical absorption coefficient increases sharply at lower wavelength, which is a sign of the narrower size distribution of the ZnS nanoparticles. Blue shifting of the absorption peak (absorption edge at about 325 nm) is due to quantum confinement of the excitons present in the sample resulting in a more discrete energy of the spectrum of individual nanoparticles. Optical excitation of electrons across the band gap is a strongly allowed transition, and this causes a sharp increase in the absorbance at the wavelength corresponding to the optical energy band gap.
The fundamental absorption, which corresponds to electron excitation from the valance band to conduction band, can be used to determine the value of the optical band gap of the ZnS nanoparticles. The optical band gap of as-prepared ZnS nanoparticles was determined using absorption spectra by the following Tauc relation [
Plot of
The value of the optical band gap (
Plot of
The average particle size of ZnS nanoparticles has also been determined by using the mathematical model of “effective mass approximation.” According to “effective mass approximation” (EMA) for spherical clusters [
The values of the effective mass of an electron and hole for ZnS are
ZnS nanoparticles were effectively synthesized by an infrared (IR) radiation assisted and Stokes’ law based new controlled bottom-up approach without using any capping agent or stirring. The reaction is controlled by the IR radiation surface heating and consequently the synthesis mechanism is based on the physics of the well-known Stokes’ law for a free body falling in a quiescent and viscous fluid. The structural and optical characterization of the as-prepared ZnS nanoparticles confirms the nanosize formation. The XRD pattern also confirms that the ZnS nanoparticles are polycrystalline in nature having a cubic (zinc blende) lattice structure. The absorption spectra show the quantum confinement effect which was used in the studies of the optical properties of the as-synthesized ZnS nanoparticles. The initial results of this study show the importance of their synthesis approach and further work to expose the control of size, shape, and orientation of the nanoparticles by optimizing the process parameters is in progress.
The authors declare that there is no conflict of interests regarding the publication of this paper.