In this paper, ZnS one-dimensional (1D) nanostructures including tetrapods, nanorods, nanobelts, and nanoslices were selectively synthesized by using RF thermal plasma in a wall-free way. The feeding rate and the cooling flow rate were the critical experimental parameters for defining the morphology of the final products. The detailed structures of synthesized ZnS nanostructures were studied through transmission electron microscope, X-ray diffraction, and high-resolution transmission electron microscope. A collision-controlled growth mechanism was proposed to explain the growth process that occurred exclusively in the gas current by a flowing way, and the whole process was completed in several seconds. In conclusion, the present synthetic route provides a facile way to synthesize ZnS and other hexagonal-structured 1D nanostructures in a rapid and scalable way.
KeywordsZinc sulphide 1D nanocrystal Thermal plasma
In the past decade, considerable effort has been paid on the preparation of 1D nanostructures such as nanorods, nanowires, nanobelts, and nanotubes due to their potential application as building blocks for constructing a range of electronic and photonic nanodevices (such as nanolasers, nanosensors, field-effect transistors, and nanocantilevers, etc.) [1–8]. Among these materials, 1D sulphide nanostructures have attracted great interest due to their sizes and morphology-related properties and their emerging applications in functional nanodevices. For example, in nanocomposite-based photovoltaics, 1D sulphide may provide paths for more efficient transport of carriers from photoinduced interfacial charge separation, and thus increase the overall photocurrent efficiency [4, 9]. Accordingly, 1D sulphide materials are not only fundamentally interesting but also highly promising in a broad range of applications and merit extensive investigation.
ZnS is an important II–VI group semiconductor compound with a direct band gap of 3.7 eV that exhibits wide optical transparency from the visible light (0.4 μm) to the deep infrared region (12 μm), which makes ZnS as one of the most common materials used in optical and optoelectronic fields [10–12]. For example, ZnS is widely used to fabricate the optical windows due to its excellent optical transparency together with the chemical and thermal stability [13, 14]. In recent years, nanoscale structures of ZnS, particularly the quasi 1D nanostructures, have been the focus of intensive research used as building blocks for nanoelectronic and nanophotonic systems [15–20]. Because of their enhanced properties significantly different from those of their bulk counterparts and the new nanostructure-based devices based on these properties, synthesis and fabrication of ZnS 1D nanostructures have been the focus of increasing research, and many methods have been developed to synthesize ZnS 1D nanostructures [21–24]. Among these methods, thermal evaporation has been confirmed to be a simple way to obtain ZnS 1D nanostructures with different morphology. Naoto et al. , reported the synthesis of ZnS nanostructures by simple thermal evaporation of ZnS powder in the presence of Au catalysts at 970 °C, and in their case, well-controlled wurtzite ZnS nanobelts, nanosheets, and nanorods were obtained. More complex structures, such as hierarchical structured nanohelices, have also been synthesized by the thermal evaporation process in Wang’s laboratory using zinc sulfide as source material . Because the supersaturation plays a key role in the morphology of final products in vapor synthesis, the products with different shape were often obtained at different temperature zone of the same substrate, which made an obstacle to separate and collect of the products with single shape. In addition, the main limitation of their methods is the small-scale quantities with the yields no more than gram level. The long reaction time and low process pressure were the main factors that restrict these methods to scale up.
In our previous report, uniform ZnO nanorods with closely controlled aspect ratio have been successfully synthesized by the RF thermal plasma beyond gram level per minute without using any catalysts, but the products with other shape (such as nanobelts, nanowires) could not be obtained . Because the zinc and sulfur could be vaporized at a relatively low temperature due to their low vapor formation points at 1,180 K for Zn and 718 K for S, respectively, it is much easier and economical to fabricate ZnS nanostructures based on the reaction between the vapors of Zn and S. In this paper, we reported that the synthesis of ZnS 1D nanostructures in a scalable way using Zn and S powder as the starting materials, and the product shape could be well controlled by adjusting the experiment parameters. The plasma synthesis process has been proved as a facile way to obtain 1D nanostructures.
In the present study, a RF thermal plasma reactor operated at 30 kW was used to synthesize ZnS nanocrystals. The plasma was generated by a three-turn, water-cooled induction coil from a RF power-supply system with a frequency of 4 MHz. In the experiment process, the starting materials of commercial Zn and S powder (-200 mesh) were firstly mixed together (according to the mole ratio of 1:1) and then fed into the plasma through a water-cooled atomizer probe by the carrier gas (nitrogen gas, at a flow rate of 150 L/h) in a continuous way. The raw materials subsequently underwent vaporization, nucleation, and growth processes, and the final products were collected at the bottom of the collector. Argon (1.0 m3/h) and nitrogen gas (5.0 m3/h) were injected as the plasma-forming gas and sheath gas, respectively. The chamber pressure was maintained at atmospheric pressure. A schematic illustration of the configuration of the apparatus is available in the literature .
The structural characteristics of as-synthesized samples were determined using an X-ray diffractometer (XRD, Philips X’Pert PRO MPD) through the 2θ-range from 10 to 90 degree at a scan rate of 0.02 deg s−1operated at 40 kV and 30 mA with Cu Kα radiation. The morphology of the products was characterized using both a scanning electron microscopy (SEM, JSM-6700F) and transmission electron microscope (TEM, Hitachi H-800). The detailed morphology and structural characterization were investigated by a high-resolution transmission electron microscope (HRTEM) and selected-area electron diffraction (SAED) in the same transmission electron microscope (TEM, JEOL JEM-2011). Photoluminescence (PL) measurements were carried out at room temperature using 325 nm as the excitation wavelength with a luminescence spectrometer (Perkin-Elmer, LS50B).
Results and Discussions
In plasma synthesis process, the resident time of particles in the plasma system is no more than several seconds, that is, the reaction time is very short. Even for the nanobelts longer than tens of micrometers, the growth is completed in several seconds as well. One of the important features of synthesis in RF plasma system is that the growth rate is rather rapid when compared with the conventional vapor deposition process. In the experimental process, vapor species were formed due to the high processing temperature (up to 1.0 × 104 K) in the flame zone, and then cooled to form ultrahigh-level supersaturated vapor in the plasma tail, which provides intensive growth driver for ZnS to nucleate and grow. In addition, part of zinc was ionized in the plasma zone, which accelerates the transmittability of electrons from zinc to sulfur. All of these provide intensive growth driver for ZnS to nucleate and grow, and the 1D nanostructures were finally formed due to the anisotropic growth habit of ZnS crystals results from the cation- or anion-terminated atomic planes . Different materials were also synthesized by this method in our laboratory (including previous reported ZnO, Zn, AlN, and WO3).
In summary, various 1D nanostructures of ZnS, including the tetrapods, nanorods, nanobelts, and nanoslices, were successfully synthesized by the thermal evaporation of zinc and sulfur powder using a plasma system. The morphology of synthesized products could be well controlled by varying the feeding rate and the cooling flow rate, and the synthesized crystals display uniform width all less than 100 nm and the length varying from several hundred nanometers to micrometers. The formation mechanism of 1D nanostructures was due to the anisotropic growth habit of ZnS crystal, and the rapid growth rate of plasma synthesis process also caused the formation of crystal defects in the products. In addition, this method provides a facile way to synthesize ZnS and other wurtzite-structured 1D nanostructures in a scalable and continuous way.
This work was supported financially by the National High Technology Research and Development Program of China (863) (No. 2008AA03Z308).