Figure 1. During microwave irradiation, the PVP may be broken to a limited extent, thereby producing shorter polymer chains that reduce the capping function on the surface of the metallic ions. Since the electric component of the microwave irradiation causes heating by two main mechanisms: i dipolar polarization of water and ii ionic conduction of precursors, introduction of the polymer causes restriction of the rotational motion of dipole molecules and immobilization of ions.
These factors affect the heating rate and consequently the rate of nucleation and subsequent growth of the ZnS-NPs. In a one-pot microwave-based synthesis, the relative extent of the precursor consumption between the nucleation and growth stages will dictate the final nanoparticle size, for a given amount of precursor introduced into the reaction medium.
The reason for this is that particles can only grow until all of the molecular precursor is consumed Figure 2. Figure 2. Schematic representation of the nucleation and growth of nanoparticles in the absence of Ostwald ripening.
Fast nucleation results in a high particle concentration and ultimately yields small particles, while a slow nucleation results in a low concentration of embryonic seeds that consume the same amount of precursor and therefore results in a population of proportionally larger particles Figure 2 [24,25]. The peaks observed in the XRD patterns at 20 values of Furthermore, it is clearly observable that the presence of PVP attenuates the intensity of the XRD peaks compared with the absence of the polymer and all peaks shift slightly towards a higher 26 value.
This may be attributed to a lower degree of crystallinity and internal stress , which clearly supports the formation of PVP-capped ZnS-NPs. Broad diffraction peaks are typically attributed to a smaller particle size. The crystalline size of the ZnS-NPs was calculated using the reflection of the XRD patterns, based on the Scherrer Equation [3,27] according to: k X P cos 0 where D is the average crystallite size, k is particle shape factor, X is the X-ray wavelength 0.
The estimated sizes are 4. Figure 3. XRD pattern of the ZnS nanoparticles synthesized with A thioacetamide or B sodium sulfide as sulfur source in the absence and presence of polymer. The results show that both the presence of PVP and the PVP concentration significantly affect the size distributions and dispersion of the resulting nanoparticles. Figure 4 shows the TEM images and corresponding size distribution histograms of ZnS-NPs synthesized using thioacetamide at different concentrations of polymer solution.
Consequently, nanoparticles with a small average size nm are produced, whilst the hydrogen bonds between the CH3COO- anions promote aggregation , which is clearly noticeable in the TEM image in Figure 4A. Figure 4. Similar experiments using an alternate sulfur source, i. Overall, the effect of the PVP concentration on the particle size and size distribution was more pronounced in our experiments when using sodium sulfide compared to thioacetamide.
Consequently, our expectations that the sulfur source might considerably affect the reaction process and the final nanoparticles product were effectively met. Conversely, with increasing PVP concentration, more ions are covered by the polymer, which slows the reaction process down; as a result, the size of the nanoparticles increases and monodispersed particles are obtained.
This concentration provided the conditions required to fabricate well dispersed ZnS-NPs, whilst obtaining a nanoparticle population with a sufficiently small average size and narrow size distribution. Figure 5. Furthermore, note that the C-H peak at cm-1 is significantly weakened. The peak shifting towards lower wave numbers that correspond to C-N bonds is likely due to chemical coordination of ZnS-NPs with these bonds. Figure 6.
Similarly, the nanoparticle product using sodium sulfide as a sulfur source was evaluated and the results of the FTIR measurements are presented in Figure 7. Figure 7. It should be noted that with increasing polymer concentration, the extent of the C-N peak shift decreases.
This is due to the steric effect exerted by pyrrolidone that is more prominent with increasing polymer concentration and influences the coordination between N and the ZnS-NP surface. To determine the size and nature of the bandgap, UV-vis spectra were recorded, as presented in Figure 8. The samples show a strong absorption below nm and a blue shift of the absorption edge compared to bulk ZnS onset is at nm  , which can be attributed to the quantum confinement effect.
The optical band gaps were determined from the extrapolation, as indicated by the dotted lines in Figure 8, and the results are listed in Table 1. Figure 8. The first term mathematically describes the confinement effect, whilst the second term is the Coulomb term. In a strong confinement, as in the present case, the second term is small and may be neglected . From Equation 3, the bandgap and the particle sizes were calculated see Table 1. It can be seen from the table that the particle sizes are in fair agreement with those determined from the TEM images Figures 4 and 5.
Table 1. Finally, to ascertain the stability and the shelf-life of the PVP-capped ZnS-NPs, UV-vis spectra of the various samples were re-recorded after one month storage under normal conditions ambient temperature in the dark. Stability measurements show that PVP-capped ZnS-NPs remain stabile after prolonged storage, with no significant changes in the absorption spectra.
In contrast, particles produced in aqueous solution only reach moderate stability and deteriorate over time to form significantly instable colloidal solutions, which was confirmed through TEM evaluation of stored solutions. In general, the results of our stability measurements show that a good measure of colloidal stability is present and retained. Experimental Section The starting materials for the synthesis of ZnS nanoparticles were zinc acetate, poly-A-vinylpyrrolidone PVP10 , thioacetamide, and sodium sulfide.
In a typical procedure, the appropriate amount of PVP 1 g was dissolved in 10 mL distilled water. Subsequently, 5 mM zinc acetate was added to the PVP solution under stirring rpm and the resulting solution was homogenized for 30 min. Next, 10 mL of aqueous sulfur source solution was added and stirred until a well-dissolved solution was obtained.
The concentration of the sulfur sources was slightly higher than the zinc source in order to ensure that the reaction would proceed to completion. The precipitates were centrifuged rpm, 5 min and washed several times with distilled water and ethanol. Conclusions Well-dispersed ZnS-NPs with narrow size distributions were prepared in a polyvinyl pyrrolidone polymeric solution via a simple, rapid and energy efficient microwave synthesis method.
Since the majority of atoms in a nanoparticle are located on its surface, the modification of the nanoparticle's surface has been recognized as the prime method to tailor nanomaterials for particular applications, with a high measure of control over the final product. Differences in the ZnS-NP size and dispersion were found to be polymer concentration dependant, as clearly visible upon TEM microscopic evaluation.
Therefore, capping with a polymer, such as PVP, occurs via conjugation between PVP and the ZnS-NPs and not only offers enhanced colloidal stability, but also some measure of control over the size and the size distribution. These results also indicate the significance of the sulfur source and thus the reaction mechanism on the final product.
Figure 9. Interaction of PVP with the metal ions located at the nanoparticle's surface can give rise to overlap of the molecular orbitals of PVP with the atomic orbitals of the metal ions. In conclusion, the present work presents an easy, inexpensive and effective way to produce well-dispersed, small, and stabile nanoparticles with a polymer coating that allows easy further functionalization and affords potentially controllable physical, especially optical properties.
The authors would also like to thank the staff of the Faculty of Science and the Bioscience Institute of the University Putra Malaysia, who contributed to this work. References 1. Yi, G. Nanocrystalline Phosphors.
Ghosh, G. He, R. A , , Wang, L. Preparation and characterization of the ZnS nanospheres with narrow size distribution. Kwiatkowski, K. Lukehart, C. Wang, X. Large-scale synthesis well-dispersed ZnS microspheres and their photoluminescence, photocatalysis properties. Ni, Y. Rapid fabrication and optical properties of zinc sulfide nanocrystallines in a heterogeneous system. The observed photoluminescence peak at nm is feature in all the PL spectra that the emission maxima markedly blue shifted relative to that of the bulk ZnS are markedly blue shifted relative to that of the bulk nm.
It is to be noted that the intensity of the peak ZnS which occur at about nm. The PL peak at nm is assigned to transi- tions involving interstitial zinc or sulphur atoms . The emission peak is strongly blue shifted 33 nm compared to that of bulk c c 50 ZnS. Wavelength nm Fig. In Fig. Intensity a. Molecular structure of PVP. Here, we propose Fig.
It is found that the molar ratio 0. When this ratio is less We conclude that the developed soft chemical tech- than 0. This sivate the in situ generated ZnS quantum dots. Acknowledgments Authors thank Dr. References  A. Henglein, Chem. Nirmal, L. Brus, Science Bruchez Jr. Moronne, P. Gin, S. Weiss, A. Alivisatos, c Science Chan, S. Nie, Science Trindade, P. Azad Malik, P. Revaprasadu, J. Chatterjee, A. Patra, J. Arul Dhas, A.
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Solids , 64, Figure 6. Aspects
Furthermore, note that the C-H peak at cm-1 is significantly weakened. Further, it is to be pointed out that the 0 cluster size of ZnS estimated from the optical absorption Wavelength nm study are comparable to those obtained from the XRD analysis, although in some cases minor deviations exist. Weiss, A.
Phase controlled monodispersed cds nanocrystals synthesized in polymer solution using microwave irradiation. Experimental particles, the same precursor composition, in the ab- sence of PVP, was used. Ghosh et al. Nie, Science Baghbanzadeh, M.
Fluorescence dispersed sulphide nanoparticles were collected from spectra of ZnS nanoparticles dispersed in water con- the DMF solution by adding a known volume of ace- ductivity of water: 1. Table 1. Intensity a.
Yan, Z. This re- sult reveals the importance of concentration of PVP. Zhao, Q. Zhe, Colloids  J.
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Homogeneous ZnS coating onto TiO2 nanoparticles by a simple one pot sonochemical method. However, the excitonic absorption peak at nm is observed for PVP capped Intensity a. A , , Ni, Y.
A , , A y-ray irradiation route to fabricate monodisperse zinc sulfide hollow spheres using silica as templates.
Figure 6. A major advantage of the microwave synthesis method is that it provides homogenous internal and volumetric heating at rapid rates through interaction of the dipole moment or molecular ionic with electric and magnetic fields . The peak shifting towards lower wave numbers that correspond to C-N bonds is likely due to chemical coordination of ZnS-NPs with these bonds.
The concentration of the sulfur sources was slightly higher than the zinc source in order to ensure that the reaction would proceed to completion. Effect of capping term is small and can be neglected. A major advantage of the microwave synthesis method is that it provides homogenous internal and volumetric heating at rapid rates through interaction of the dipole moment or molecular ionic with electric and magnetic fields . Brus, Science Kalasinsky, M.
Moronne, P. Synthesis, structural and optical properties of CeO2 nanoparticles synthesized by a simple polyvinyl pyrrolidone PVP solution route.
Gin, S. The introduction of the polymer affects the heating rate by restriction of the rotational motion of dipole molecules and immobilization of ions. Particles were collected using USA in the wavelength range of — nm. These results also indicate the significance of the sulfur source and thus the reaction mechanism on the final product.