Diamond anvils lead to first chemical reactions by mechanical pressure

Materials World magazine
1 Apr 2018

The use of diamond molecular anvils in the first chemical reactions by mechanical pressure could lead to a new field of mechanosynthesis. Khai Trung Le reports.

Using the smallest possible forms of diamond and other super-hard material as molecular anvils has lead to the world’s first chemical reactions triggered by mechanical pressure alone, with the potential to lead to more efficient and precise chemistry.

Researchers from Stanford University, USA, and the US Department of Energy SLAC National Accelerator Laboratory, formerly named the Stanford Linear Accelerator Centre, used diamond and other super-hard specks as anvils to squeeze and twist molecules until chemical bonds break and atoms exchange electrons. Hao Yan, lead author and Stanford Physical Science Research Associate, told Materials World, ‘Unlike other mechanical techniques, which basically pull molecules until they break apart, we show that pressure from molecular anvils can both break chemical bonds and trigger another type of reaction where electrons move from one atom to another.’ The process does not require additional heat or solvents, leading to greater energy efficiency and less environmental impact.

Detailed in the paper, Sterically controlled mechanochemistry under hydrostatic pressure, published in Nature, the team use a diamond anvil cell roughly the size of an espresso cup that squeezes materials between the flattened tips of two diamonds in over 500 gigapascals of pressure. Co-author and SLAC Associate Professor Wendy Mao said, ‘Pressure is force per unit area, and we are compressing a tiny amount of sample between the tips of two small diamonds that each weigh only about a quarter of a carat. So you only need a modest amount of force to reach high pressures.’

As the diamond is transparent, light can be used to examine the sample. By observing the scattering or absorption of beams the nature of the reaction can be determined. If a sample has uniform bonds, it will deform uniformally. However, the team speculated that the bonds in a material comprising hard and soft components would break down at different rates.

He tested the supposition with copper sulphur clusters consisting of eight atoms attached to molecular anvils made up of carborane. When squeezed in the diamond anvil cell, not only did they observe the atomic bonds in the cluster breaking, but the electrons moved from the sulphur atoms to the copper atoms, forming pure crystal, a reaction that would not have occurred if driven by heat. Pressure from the diamond anvil cell moved the molecular anvils, which squeezed chemical bonds in the clusters in an uneven fashion, causing bonds to break and electrons to move.

Mao said, ‘This opens up a whole new field. We’re interested in looking at how pressure can affect a wide range of technologically interesting materials such as superconductors that transmit electricity with no loss to halide perovskites, which have a lot of potential for next-generation solar cells. Once we understand what’s possible from a very basic scientific point of view we can think about the more practical side.’

Yan added, ‘If we want to dream big, could compression help us turn carbon dioxide from the air into fuel, or nitrogen from the air into fertiliser? These are some of the questions that molecular anvils will allow people to explore.’

You can read Sterically controlled mechanochemistry under hydrostatic pressure at go.nature.com/2FGfoPX