Hydrazine-assisted preparation of ZnS nanocrystals using N-acetyl-L-cysteine as capping agent

Abstract

Using N-acetyl-L-cysteine (NAC) as capping agent and different amounts of hydrazine hydrate as growth promoter, in this paper, we report a simple way for preparation of aqueous-based ZnS nanocrystals (NCs). The small-sized and well-dispersed NCs with cubic zinc blende structure and size-dependent optical behavior have been characterized by XRD, TEM, FT-IR, UV-Vis, and photoluminescence (PL) measurements. Disappearance of the thiol-related peak in the FT-IR spectrum of NAC-capped ZnS NCs demonstrated the well-passivation of ZnS NCs by deprotonated NAC molecules. Variation of optical properties of ZnS NCs has been studied at different hydrazine amounts and refluxing times. The growth-assisting characteristic of hydrazine hydrate was demonstrated by redshift in absorption edge as the hydrazine amount increased from 5 to 20 mmol, and ensuing decrease in band gap energy of ZnS NCs from 4.47 eV to 4.0 eV. On the other hand, increasing the refluxing time duration up to 20 h, resulted in obvious redshift in absorption edge and subsequent decrease in the band gap energy from 4.56 eV to 3.85 eV. The optical estimation on the size of the as-prepared ZnS NCs was obtained through effective mass approximation. Results indicated that all the NCs are small enough near the excitonic Bohr radius of ZnS which revealed the presence of strong quantum confinement effects. Effect of these parameters on PL emission intensity of ZnS NCs was studied and it was observed that the best emission result is at the presence of 12.5 mmol hydrazine hydrate and during 16 h heating. Finally, analyzing chemical stability of the as-prepared ZnS NCs against H2O2 corrosion demonstrated a well-accepting stability of emission intensity even after 108 min.