The synthesis was performed under different feeding rate along with different cooling flow rate to investigate the influence of experimental parameters on the morphology of final products, and the morphology of synthesized products was investigated by SEM as shown in Fig. . Figure a illustrates the products obtained at a feeding rate of 12 g/min, which reveals the formation of large-scaled tetrapod ZnS with uniform morphology in the products, and no particles were produced. The inset show exhibits the detailed morphology of a single tetrapod ZnS, which indicates that as-synthesized crystal consists of four rod-shaped tetrahedrally arranged legs connected at the center, forming a tetrapod structure. The length of each leg was about 200 nm, and the diameter was about 80 nm. When the feeding rate of the starting materials was increased to 24 g/min, the products mainly consist of rod-like nanostructures as shown in Fig. b, and the diameter of the synthesized nanorods was about 80 nm, and the length was beyond microns. However, the uniform nanorods or tetrapod ZnS could not be obtained if we further increase the feeding rate of starting materials. A change of morphology was observed at a particular feeding rate of 21 g/min by introducing the cooling gas (nitrogen gas) to the system. When the flow rate of cooling gas was about 3 m3/h, instead of the tetrapods and nanorods, uniform nanobelts with width about 50 nm were formed, and their lengths varied from a few tens to a hundred micrometers as shown in Fig. c. When the flow rate of the cooling gas was further increased to 6 m3/h, quadrate nanoslices with a length of about 100 nm were dominant in the final products (Fig. d), which is due to the low growth temperature caused by the excessive cooling gas that restricts the growth of crystals along their extended direction.
SEM images of ZnS nanostructures produced with different sets of feeding rate and cooling gas flow ratea12 g/min and without cooling gas,b24 g/min and without cooling gas,c21 g/min and with cooling gas 3 m3/h, andd21 g/min and with cooling gas 6 m3/h
Figure reveals the XRD patterns of products synthesized at different experimental parameters, and we can conclude that the hexagonal wurtzite ZnS with the lattice constantsa = 3.82 Å andc = 6.25 Å was obtained in all cases, and no diffraction peaks corresponding to Zn and S are detected in the XRD patterns. The results demonstrate the formation of well-crystallized ZnS nanostructures, but there still have three weak diffraction peaks indexed to wurtzite ZnO detected (as marked * in the XRD pattern) in the sample, which reveals the formation of trace amounts of ZnO in the synthesized process. This is because that the nitrogen gas used in our experiment contains about 0.5% content of oxygen gas, and the oxygen has higher activity than sulfur at high temperature. When the starting materials were supplied into the plasma flame, the reaction between zinc and oxygen is prior to zinc and sulfur attributed to the thermodynamics reasons. As a result, ZnO was formed prior to ZnS. Consequently, the products we obtained were wurtzite ZnS with little amount of ZnO.
Figure 2 XRD patterns of ZnS nanostructures produced with different sets of feeding rate and cooling gas flow ratea12 g/min and without cooling gas,b24 g/min and without cooling gas,c21 g/min and with cooling gas 3 m3/h, andd21 g/min and with cooling gas 6 m3 (more ...)
The morphology and structure of synthesized ZnS products were also studied by the TEM analysis. Low-magnified TEM images (the results without show) of the products synthesized at different experimental parameters further proved that the crystals consist of different 1D nanostructures as the SEM images shown in Fig. . Figure shows the magnified TEM images and their corresponding HRTEM images of single ZnS 1D nanostructures, respectively. Magnified TEM images of synthesized single crystals of tetrapod, nanorod, and nanobelt were shown in Fig. a–c, from which we can see that the synthesized crystals display uniform width all less than 100 nm along their axes, while the length varied from several hundred nanometers to micrometers. Figure d–f illustrates the HRTEM images of corresponding nanostructures shown in Fig. a–c, revealing their perfect hexagonal wurtzite structure of ZnS. The measured spacing of the crystallographic planes is about 0.625 nm, corresponding to the (001) lattice planes. The corresponding SAED taken perpendicular to the axis of the nanostructures is shown in the inset of Fig. d–f respectively. It, together with the HRTEM, confirms that the growth of single crystalline ZnS tetrapods and nanobelts was generally along  direction, while the nanorods was perpendicular to  direction. Although the synthesized nanostructures prefer to grow along different growth directions, they are all hexagonal elongation along thec-axis according to the theoretical and crystal habit of the ZnS.
Magnified TEM images of synthesized single ZnSatetrapod,bnanorod,cnanobelt, and corresponding HRTEM shown in (d), (e), and (f)
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 [26
]. Different materials were also synthesized by this method in our laboratory (including previous reported ZnO, Zn, AlN, and WO3
Meanwhile, rapid growth rate sometimes may cause the formation of crystal defects in crystal lattice during the growth process, and Fig. confirms the formation of two different crystal defects in the final products. Figure a illustrates the stacking fault existed in a single nanobelt, which is parallel to the axis and runs through out the nanobelt. Figure b shows a leg of tetrapod ZnS with two types of structure zone, one consists of the wurtzite structure (hcp, hexagonal close-packed) in zone a and the other is sphalerite structure (fcc, face-centered cubic) in zone b, and the two different structures were formed by changing the stacking sequence of the closed-packed planes of the ZnS crystal and resulted from the crystallogenesis of ZnS [28
The crystal defects exist in the ZnS nanocrystalsastacking fault in a nanobelt andbthe sphalerite structure and the wurtzite structure polytypes in a leg of tetrapod
Interestingly, some hollow nanocrystals were obtained when we analyzed the products collected from the inner wall of the reactor. Figure shows the TEM images of obtained hollow tetrapod and tubular nanocrystals, which are resulted from the oxidation process of the ZnS nanocrystals by the atomic oxygen at high temperature. Loh et al. [29
] reported the hollowing mechanism of zinc sulfide nanowires by an atomic oxygen beam, and the oxygen atoms play key role in the formation of hollow structures. In our experiment, low concentration of oxygen gas was introduced into the reaction chamber continuously due to the impurity of the nitrogen gas. The oxygen gas was then heated to an ultra-high temperature and partially atomized even ionized in the plasma flame due to the effect of plasma electromagnetic field, which made oxygen possess high activity. When the atomized oxygen collided with the crystals attached on the reactor wall under high temperature, the substituted reaction of S by O was occurred on the surface of the crystals, and the inner S was slowly diffused outward. Eventually, hollow nanostructures were formed after the ZnS core was removed completely, and EDX analysis also proved this point.
TEM images of synthesized hollow nanostructuresatetrapod andbnanotube
In the conventional vapor-solid (VS) process, evaporation, chemical reduction, or gaseous reaction first generates the vapor. The vapor is subsequently transported and condensed onto a substrate, and then the nuclei are formed, continually absorbed the arrived vapor species and finally grew into 1D structures. The growth process of crystals occurs at an immovable spot, and the substrate plays a key role in the formation of 1D nanostructures. While in the plasma synthesis, this process is quite different. There is no settled spot for nuclei to deposit and grow. The starting materials are vaporized in the flame, and then undergo condensation, nucleation, and growth processes exclusively in the flowing gas current. The previously condensed vapor cannot complete the growth process by continually absorbing the latterly arrived vapor species because when the later formed vapor species arrived, the previously formed nuclei have already moved away in their falling process. Therefore, the nucleation and growth processes could only complete by colliding of different nuclei around themselves. By this way, the condensed vapor absorbs other species and energy, completes the nucleation and growth process in the falling process, and finally forms the 1D nanostructures. Figure shows HRTEM image of synthesized ZnS tip, and there is no Zn metal or catalyst particles found on the tip, which further confirms that the growth process in the plasma is still a VS mechanism. Analyzing the flow conditions in our experiment reactor, it could be concluded that distribution of the stream lines in plasma reactor is mostly uniform by calculating the Reynolds’s number, which ensures the uniform environment for different crystal to nucleate and grow. Accordingly, uniform products with singly shape were obtained in our experiment.
HRTEM image of one tip of synthesized ZnS nanorods
The PL measurement of the ZnS nanostructures was carried out with a Xe lamp at 325 nm excitation at room temperature. PL spectrum taken from the ZnS nanorods, nanowires, and tetrapods gives almost similar emission behavior, as shown in Fig. . Three strong and broad emission bands located at 367, 453, and 487 nm have been observed. It is well known that the luminescent peak centered at 367 nm could be assigned to the UV-excitonic emission [14
]. The ZnS nanomaterials reported previously have PL emission with bands in the range 400–450 nm [30
] associated with the sulfur vacancy. Therefore, the strong blue emission at around 453 nm should be assigned to the stoichiometric vacancies in the ZnS nanocrystal, and the nanobelts present high sulfur vacancy than the nanorods and tetrapods according to intensity of the emission peaks. Another strong emission peak is located at 487 nm, which is similar to that of the well-known ZnS-related luminescence (at about 480 nm) produced by zinc vacancies [32
]. Because the crystal growth in the plasma process is very rapid and the structure defects are inevitably, we reasonably believe that the green–blue luminescence is associated with the defect-related emission of the ZnS host.
PL spectra of synthesized ZnS nanostructuresananobelts,bnanorods, andctetrapods