The optical bandgap of the nanoparticles was obtained from the UV absorption stud, which corresponded to the transition from the valance band to the conduction band, using the following equations:
​
​
​
where E is photon energy, h = 6.6261 x 10 J s is Planck’s constant, c = 2.9979 x 108 ms is speed of light, λ is the wavelength of corresponding light wave.
​
A=εcl
​
where A is absorbance of the sample, ε is absorption coefficient, c is concentration of solution, L is path length which is equivalent to width of cuvette.
​
αhv=A(hv-Eg)
​
where α is the absorption coefficient, hv is the incident photon energy, A is a constant, Eg is the optical band gap energy of the material. n is ½ for direct transition.
The band gap of ZnS nanoparticles was determined by plotting (αhv) against hv, then extrapolating the straight line portion to (αhv) = 0 (tauc plot).
The UV-Vis spectra was obtained in a wavelength range of 500 nm - 200 nm to obtain the optical bandgap energy. The excitation absorption peak of the synthesized ZnS nanoparticles appeared to be at around 330 nm. No peaks were observed between 500 nm to 800 nm.
Results
UV-Vis spectrum of ZnS:Cu nanoparticles
Optical bandgap of ZnS:Cu nanoparticles
Optical bandgap of ZnS:Cu+Ag nanoparticles
UV-Vis spectrum of ZnS:Cu+Ag nanoparticles
Optical bandgap of ZnS:Cu+Y nanoparticles
UV-Vis spectrum of ZnS:Cu+Y nanoparticles
HCI
UV-Vis spectrum of ZnS:Cu nanoparticles
Optical bandgap of ZnS:Cu nanoparticles
Bugil
UV-Vis spectrum of ZnS:Cu+Al nanoparticles
Optical bandgap of ZnS:Cu+Al nanoparticles
UV-Vis spectrum of ZnS:Cu+Mn nanoparticles
Optical bandgap of ZnS:Cu+Mn nanoparticles
n
2
-1
-34
2
UV-Vis Spectra and Optical Bandgap of ZnS nanoparticles
Kruskal-Wallis statistical test was then used to determine whether there was significant difference in bandgaps between distinct samples of ZnS nanoparticles. The null hypothesis assumes that all bandgaps are the same regardless of metal dopants. The alternative hypothesis suggests that at least one bandgap is significantly different from the rest. The level of significance, α, was set to 0.05.
​
Figure 21: Graph showing effect of dopants on bandgap
The p-value was found to be 0. Since p < α, the null hypothesis was rejected and there is significant difference in bandgaps for different dopants. Based on the trend of the bandgaps, we hypothesise that the larger the ionic radius of the metal dopants, the larger the particle size and hence the smaller the bandgap.
