Growth of nanocrystals (NCs) via the oriented attachment (OA) mechanism is based on the spontaneous self-organization of adjacent nanocrystals into a single solid particle that share a common crystallographic orientation and often with abrupt edges. Pyrite iron disulfide (FeS) has been intensively explored for applications in photovoltaic devices, photoeletrochemical solar cells, cathode material for lithium batteries, and a depolarizer anode for hydrogen production.

Here, we report the growth of anisotropic iron pyrite NCs following the OA mechanism at the early growth stage of the hot injection synthesis. The time-dependent X-ray diffraction (XRD) patterns show the pyrite phase NCs are formed even within a very short time after the reaction taking place. The time-dependent transmission electron microscope (TEM) images indicate that the formation of anisotropic iron pyrite NCs involves a nucleation burst and an attachment process at the early growth stage. High-resolution TME (HRTEM) images reveal the OA attachments including two primary NCs, one primary NC with a dimer or a trimer, or multiple primary NCs.

By analyzing the HRTEM images, it was found that the attachments tend to occur at the {100} and {210} facets under the sulfur-rich condition. After considering the atomic structures as well as the surface energies of different termination of {100} and {210} surfaces, we propose that the attachments could occur through a primary NC with the {210}-Fe or {001)-Fe facet to another proximately located primary NC with the {210)-S or {001}-2S, respectively. Faceted anisotropic iron pyrite NCs with {100}, {210} and {111} facets and better crystallinity are obtained with longer reaction time.

This study opens a way to better understand the growth mechanism of anisotropic iron pyrite NCs and to rationally manipulate the morphology of iron pyrite NCs in order to achieve desired physical properties of iron pyrite NCs for efficient photovoltaic conversions.

Leize Zhu

University of Washington

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