Abstract
Copper chalcogenide nanocrystals find applications in photovoltaic inks, bio labels, and thermoelectric materials. We reveal insights in the nucleation and growth during synthesis of anisotropic Cu2ZnSnS4 nanocrystals by simultaneously performing in situ X-ray absorption spectroscopy (XAS) and small-angle X-ray scattering (SAXS). Real-time XAFS reveals that upon thiol injection into the reaction flask, a key copper thiolate intermediate species is formed within fractions of seconds, which decomposes further within a narrow temperature and time window to form copper sulfide nanocrystals. These nanocrystals convert into Cu2ZnSnS4 nanorods by sequentially incorporating Sn and Zn. Real-time SAXS and ex situ TEM of aliquots corroborate these findings. Our work demonstrates how combined in situ X-ray absorption and small-angle X-ray scattering enables the understanding of mechanistic pathways in colloidal nanocrystal formation.
Highlights
Colloidal nanocrystal (NC) syntheses have progressed to the point where excellent control of size and shape allows for a multitude of applications that utilize carefully tuned electronic and optical properties such as displays, biotags, and photovoltaics.[1]
All recorded X-ray absorption spectra were fitted with a linear combination of reference spectra taken from the reaction itself
By means of this internal linear combination analysis (LCA), four reference points are identified on the Cu K-edge during the reaction: (1) before injection, where metal salts are in solution; (2) after the injection of the anion precursor (1st intermediate); (3) a second reaction intermediate; and (4) the final state, consisting of CZTS nanorods
Summary
Colloidal nanocrystal (NC) syntheses have progressed to the point where excellent control of size and shape allows for a multitude of applications that utilize carefully tuned electronic and optical properties such as displays, biotags, and photovoltaics.[1]. This allows for compositionally tunable optical band gaps with high optical absorption coefficients and good photostability.[11−15] In addition, the natural abundance and relatively low toxicity of these elements makes them suitable for a more sustainable chemistry.[11,16] In this colloidal NC system, the presence of three different metal cations in combination with a chalcogen anion requires judicious selection of precursors, solvents, and ligands in combination with optimized reaction protocols to deliver nearly monodisperse particles.[17,18] we have shown that these particles can be stabilized in wurtzite form that is not stable in the bulk, allowing anisotropic control for the formation of nanorods with defined aspect ratio.[19−23]
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