Controlling Crystal Shape at the Nanoscale

Overview

This article introduces a paper on controlling the shape of pyrite, also known as fool's gold, through crystal growth.

Pyrite is a mineral that shines with a golden, cubic appearance, yet it is extremely inexpensive compared to gold.
Given its characteristic color, shape, and low cost, many mineral enthusiasts likely own a piece.

Pyrite is considered promising for industrial use as a battery electrode material. However, to actually utilize pyrite, its crystal shape must be precisely controlled.

In this study, we investigate a somewhat unique method to control crystal shape at the nanoscale to grow pyrite crystals.

The paper discussed here:
Symmetry-Defying Iron Pyrite (FeS2) Nanocrystals through Oriented Attachment,
Maogang Gong, Alec Kirkeminde & Shenqiang Ren, SCIENTIFIC REPORTS, 3 : 2092

Ostwald Ripening and Oriented Attachment (OR and OA)

Here, I will briefly introduce two fundamental crystal growth processes: Ostwald ripening and oriented attachment.

First, in general crystal growth, a phenomenon called Ostwald ripening occurs. This is a process where, when large and small crystals exist in a solution, the small crystals become smaller and the large crystals become larger.

On the other hand, oriented attachment, a less familiar term in crystal growth, is, as the name suggests, a growth process where crystals align their orientations, make contact, and merge into one. This is a phenomenon observed in very small nanoparticles.

Cited from L. Bahrig et al., CrystEngComm, 2014, 16, 9408–9424

In Ostwald ripening, atoms represented by circles in the figure above attach one by one to grow, whereas in OA growth, nanoparticles (depicted as rectangular parallelepipeds) where atoms have already gathered combine to form a single large crystal.

What was discovered?

In this study, pyrite nanoparticles were synthesized, and slightly larger crystals were produced through the aforementioned oriented attachment (OA) growth. Depending on the temperature during the initial synthesis of the nanoparticles, nanoparticles with slightly different shapes are created.

This difference in shape influences the final crystal shape of the pyrite.
Nanoparticles synthesized at low temperatures have many {100} facets, which, when they come into contact and merge, form a single large cubic crystal.
Conversely, nanoparticles synthesized at high temperatures have many {110} facets, which, when they come into contact and merge, form a single sheet-like crystal.

{100} facets and {110} facets refer to crystal planes.

This paper demonstrated the possibility of determining the final crystal shape by controlling the shape of the initial nanoparticles.
If this research progresses, it will be possible to produce crystals of desired shapes at the nanoscale, which could be applied not only to electrode materials but also to various other fields.

Impressions

OA growth is a mysterious phenomenon that is still being researched at the cutting edge today.
Furthermore, it has been shown that this OA growth also occurs in colloidal crystals where colloidal particles are arranged regularly, suggesting it may be a phenomenon observed throughout crystal growth.

I understand that atoms and colloids want to arrange themselves in an orderly fashion, but when you watch a video of particles actually sticking together, it feels as if some mysterious force is at work.