Stable aqueous colloids of ZnS quantum dots prepared using double hydrophilic block copolymers

Abstract

Stable aqueous colloids of ZnS nanoparticles have been synthesised using zinc acetate, sodium sulfide and asymmetric double hydrophilic block copolymers PAA-b-PAM (polyacrylic acid-block-polyacrylamide) with various block molecular weights: M_PAA = 1000, 3000, 5000 g mol⁻¹ and M_PAM = 10 000, 15 000, 30 000 g mol⁻¹, respectively. The synthesis involves the formation of precursor hybrid polyion complex (HPIC) micelles, in which the micelle core is composed of Zn²⁺ ions complexed by polyacrylate blocks, while the corona is constituted by the stabilising neutral PAM blocks. The addition of S²⁻ ions in the micelle solutions yields ZnS nanoparticles with a diameter of 3–4 nm. The ZnS nanoparticle size is weakly dependent on the copolymer block length, while the particle size distribution correlates with the copolymer asymmetry degree in such a way that a higher M_PAM/M_PAA ratio yields particles with a narrower size distribution. Characterisation by XRD and SEAD revealed that ZnS has a cubic sphalerite structure with the cell parameter a = 5.25 ± 0.05 Å that is smaller than that of the bulk material (a = 5.406 ± 0.05 Å). The obtained colloids present photoluminescent properties with an emission band peaking at 410 nm and exhibit a blue-shift of the band-gap energy in the UV-visible absorption spectra due to the quantum-confinement effect.