pH-induced wurtzite-zinc blende heterogeneous phase formation, optical properties tuning and thermal stability improvement of green synthesized CdS nanoparticles

This is the first paper to report on the pH response to heterogeneous wurtzite/zinc blende phase transformation, optical tunability and thermal stability advancement of the CdS nanoparticles synthesized via co-precipitation, followed by subsequent thermal treatment at a desired annealing temperature of 320 °C, while the solution pH was varied during CdS synthesis by adjusting the ammonium salt concentration. The surface morphology, crystalline structure, functional groups, optical properties and thermal stability of CdS were characterized by scanning electron microscopy (SEM), X-ray diffractometer (XRD), Fourier-transform infrared spectroscopy (FTIR), UV-visible spectrophotometer, thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC), respectively. The results show that a dominant sharp band occurs in the FTIR spectra, which authenticates the presence of Cd-S bonds. XRD results reveal that as the pH declines, CdS in the initial cubic phase has gradually transformed into a heterogeneous phase with the coexistence of cubic and hexagonal structures. As observed from the SEM images, the CdS nanoparticles display a homogeneous, smooth and spherically shaped morphology. Optical absorption characterized by UV-visible spectrophotometry denotes that the band gap decreases proportionally with pH, which could be attributed to the formation of larger grain sizes from the aggregation of many small nanocrystallites. TGA and DSC analyses demonstrate an improvement in the thermal stability of CdS with increasing pH values. Consequently, the present findings dictate that pH tunability could be a valuable approach to procuring the desired properties for the respective applications of CdS in diverse fields.