Effects of Sulfur Doping and Temperature on the Energy Bandgap of ZnO Nanoparticles and Their Antibacterial Activities

Abstract

Metal oxide nanoparticles (MO-NPs) are presently an area of intense scientific research, attributable to their wide variety of potential applications in biomedical, optical, and electronic fields. MO-NPs such as zinc oxide nanoparticles (ZnO-NPs) and others have a very high surface-area-to-volume ratio and are excellent catalysts. MO-NPs could also cause unexpected effects in living cells because their sizes are similar to important biological molecules, or parts of them, or because they could pass through barriers that block the passage of larger particles. However, undoped MO-NPs like ZnO-NPs are chemically pure, have a higher optical bandgap energy, exhibit electron-hole recombination, lack visible light absorption, and have poor antibacterial activities. To overcome these drawbacks and further outspread the use of ZnO-NPs in nanomedicine, doping seems to represent a promising solution. In this paper, the effects of temperature and sulfur doping concentration on the bandgap energy of ZnO nanoparticles are investigated. Characterizations of the synthesized ZnO-NPs using zinc acetate dihydrate as a precursor by a sol-gel method were done by using X-ray diffraction, ultraviolet-visible spectroscopy, and Fourier transform infrared spectroscopy. A comparative study was carried out to investigate the antibacterial activity of ZnO nanoparticles prepared at different temperatures and different concentrations of sulfur-doped ZnO nanoparticles against Staphylococcus aureus bacteria. Experimental results showed that the bandgap energy decreased from 3.34 to 3.27 eV and from 3.06 to 2.98 eV with increasing temperature and doping concentration. The antibacterial activity of doped ZnO nanoparticles was also tested and was found to be much better than that of bare ZnO nanoparticles.

Figure 1

Schematic representations of the synthesis procedures of ZnO nanoparticles by the sol–gel process and their antibacterial activity.

Figure 2

XRD patterns of ZnO-NPs (a) synthesized at different temperatures and (b) synthesized at varying dopant concentrations compared with undoped ZnO-NPs.

Figure 3

Ultraviolet–visible spectra of ZnO-NPs produced at different temperatures by the sol–gel method.

Figure 4

Bandgap energies of ZnO nanoparticles synthesized at various temperatures (a) 500 °C, (b) 600 °C, (c) 700 °C, and (d) 800 °C determined using the Tauc method.

Figure 5

Ultraviolet–visible absorbance spectra of synthesized ZnO nanoparticles doped with variable quantities of sulfur.

Figure 6

Bandgap energies of doped ZnO nanoparticles: (a) undoped, (b) 0.99% S/ZnO, (c) 1.96% S/ZnO, (d) 2.91% S/ZnO, and (e) 3.85% % S/ZnO.

Figure 7

FTIR bands of undoped and 0.99% S/ZnO-NP samples.

Figure 8

Zone of inhibition test for the antibacterial activity of ZnO.