Mechanistic insight into copper cation exchange in cadmium selenide semiconductor nanocrystals using X-ray absorption spectroscopy

Alex Khammang,Robert W Meulenberg,Joshua T Wright

doi:10.1038/s41467-020-20712-0

Alex Khammang, Robert W Meulenberg + Show 1 more

Open Access

https://doi.org/10.1038/s41467-020-20712-0

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Abstract

In terms of producing new advances in sustainable nanomaterials, cation exchange (CE) of post-processed colloidal nanocrystals (NCs) has opened new avenues towards producing non-toxic energy materials via simple chemical techniques. The main processes governing CE can be explained by considering hard/soft acid/base theory, but the detailed mechanism of CE, however, has been debated and has been attributed to both diffusion and vacancy processes. In this work, we have performed in situ x-ray absorption spectroscopy to further understand the mechanism of the CE of copper in solution phase CdSe NCs. The x-ray data indicates clear isosbestic points, suggestive of cooperative behavior as previously observed via optical spectroscopy. Examination of the extended x-ray absorption fine structure data points to the observation of interstitial impurities during the initial stages of CE, suggesting the diffusion process is the fundamental mechanism of CE in this system.

Highlights

In terms of producing new advances in sustainable nanomaterials, cation exchange (CE) of post-processed colloidal nanocrystals (NCs) has opened new avenues towards producing non-toxic energy materials via simple chemical techniques
Kompch and co-workers have performed the most detailed x-ray absorption spectroscopy (XAS) work on CE in CdSe18 where the mechanism of Ag CE in CdSe NC was probed via reverse Monte Carlo analysis of the extended x-ray absorption fine structure (EXAFS) data, this study focused on solid-phase materials
As the ethanolic copper salt solution is added to the NC solution, x-ray absorption fine structure (XAFS) measurements were conducted at the coordination numbers for selenium powder (Se) (Cd), Se, and Cu K-edges

Summary

Introduction

In terms of producing new advances in sustainable nanomaterials, cation exchange (CE) of post-processed colloidal nanocrystals (NCs) has opened new avenues towards producing non-toxic energy materials via simple chemical techniques. The Mn+ can substitute for a host cation via a kick-off reaction, where M+ substitutes for the host cation site (substitutionally) leading to an M− site This new negatively changed site can accumulate the interstitial Mn+ ions leading to local pockets of “MB” material. Kompch and co-workers have performed the most detailed XAS work on CE in CdSe18 where the mechanism of Ag CE in CdSe NC was probed via reverse Monte Carlo analysis of the extended x-ray absorption fine structure (EXAFS) data, this study focused on solid-phase materials. Inspired by this work and the work of the Jain group[7], in our current work, we use partial cation exchange of Cu to probe various stages of the CE reaction in solution phase CdSe NCs. Analysis of the EXAFS data show evidence for interstitials during the CE process.

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