Remarkable difference in pre-cation exchange reactions of inorganic nanoparticles in cases with eventual complete exchange

Abstract

The post-synthetic modification of inorganic nanoparticles (NPs) involving appropriate cation pairs at or near ambient conditions can exchange their spatial positions. Although the characterization of final products from these reactions has attracted much attention, little information is available on the nature of progression of these reactions in pre-cation exchange domain. This work systematically investigates the photoluminescence of the terbium cation-incorporated (doped) zinc sulfide, (Zn(Tb)S), NPs, which are post-synthetically modified by either Hg²⁺ or Pb²⁺ with reactant concentrations of [Zn(Tb)S] : [Mⁿ⁺] [M = Hg, Pb] = 1 : 10⁻¹²–1 : 10. This concentration variation with 13 orders of magnitude can be divided in four ranges. The first range comprising 1 : 10⁻¹²–1 : 10⁻⁹ neither alters the NP core structure nor has any noticeable cation specific tuning of the emission properties; the second range of 1 : 10⁻⁸–1 : 10⁻² does not impact the NP core structure, however, it can tune both the ZnS and Tb³⁺ emissions in the Zn(Tb)S NPs remarkably with the identity of post-synthetically added cation Hg²⁺ or Pb²⁺; the third range of 1 : 10⁻¹–1 : 1 is the range where cation exchange progresses, which can be visualized by both structural characterization techniques as well as photoluminescence spectroscopy; the fourth range of 1 : 1–1 : 10 is where the cation exchange is complete and can again be tracked by both structural and photoluminescence measurements. To the best of our knowledge, this study is the first of its kind that identifies the first and second ranges mentioned above. The important roles of NP surface and relative energetic alignment of co-dopants in governing the reactions are discussed. A difference in the structural architecture of the cation exchanged particles is believed to leave a signature at a very early pre-cation exchange reaction condition that can be detected by photoluminescence spectroscopy. The results obtained interestingly stress on the fact that the progression of post-synthetic modification of inorganic NPs can vary drastically even though their cation exchanged outcomes are compositionally similar and opens up avenues to sense metal ions using intermediates from the post-synthetic modification of inorganic NPs and goes beyond using the exchanged particles for such purposes.