A crystal is defined by the highly ordered, repeating arrangement of its constituent atoms. This structure dictates many of the material’s macroscopic traits, such as its strength and electrical properties. Gold (Au), known for its durability and beauty, forms crystals with a common atomic configuration. While this arrangement is stable in large pieces of metal, it leads to a variety of shapes when the material is shrunk down to the nanoscale.
The Fundamental Structure of Bulk Gold
The atomic structure of gold in its bulk form is known as Face-Centered Cubic (FCC). This internal geometry is one of the most efficient ways that identical spheres can pack together in a repeating pattern. The basic repeating unit, called the unit cell, is shaped like a cube with a gold atom positioned at each of the eight corners.
In addition to the corner atoms, a single gold atom sits in the center of each of the cube’s six faces. This arrangement results in a high packing density of about 74%. This structure can be visualized as layers of stacked spheres where the spheres in each layer nest snugly into the depressions created by the spheres below them.
Why Gold’s Crystal Structure Matters
The Face-Centered Cubic lattice is directly responsible for gold’s most recognizable physical characteristics, particularly its remarkable softness. The close-packed, high-symmetry nature of the FCC structure creates multiple “slip planes” within the crystal. These planes are specific two-dimensional layers of atoms that can slide past one another relatively easily when the material is stressed.
This feature explains gold’s extreme malleability, allowing it to be hammered into thin sheets without cracking, and its high ductility, the ability to be drawn into fine wires. The stable FCC arrangement also contributes to gold’s chemical inertness and resistance to corrosion. Furthermore, this structure facilitates the free movement of electrons, making gold an excellent conductor of electricity, surpassed only by copper and silver.
How Size Changes Gold’s Observed Shape
While the internal atomic arrangement of gold remains the stable FCC lattice, the crystal’s external shape can be altered when the material is synthesized at the nanoscale. When gold is reduced to nanoparticles, the influence of surface energy begins to dominate its behavior. At this size, scientists control the growth of different crystal faces by introducing specific chemical agents during synthesis.
These agents bind to certain planes of the FCC structure, slowing their growth while allowing other faces to expand rapidly. This selective growth results in a wide array of external shapes for the gold nanoparticles. Researchers can intentionally create:
- Spherical particles
- Elongated nanorods
- Sharp triangular nanoprisms
- Multi-pointed nanostars
- Cubic shapes
The resulting external shape dictates the particle’s functional properties. For example, the shape controls the localized surface plasmon resonance (LSPR), which is how surface electrons oscillate in response to light. This effect determines the particle’s color. Controlling the external shape is important for applications in sensing, biomedicine, and catalysis, where specific light absorption or surface reactivity is required.