Stanford researchers created and stabilised Au2+, a rare type of gold, for the first time. This elusive variant of the prized element is stabilised by a halide perovskite, a family of crystalline materials with considerable potential for solar cells, light sources, and electrical components.
Using off-the-shelf components at room temperature, Au2+ perovskite is also easy to produce.
"It was a real surprise that we were able to synthesise a stable material containing Au2+—I didn't even believe it at first," said Stanford School of Humanities and Sciences associate professor of chemistry Hemamala Karunadasa, senior author of the Nature Chemistry study published Aug. 28. "Creating the first Au2+ perovskite is exciting. Perovskite gold atoms are analogous to copper atoms in high-temperature superconductors, and heavy atoms with unpaired electrons, like Au2+, have cool magnetic effects."
"Halide perovskites possess really attractive properties for many everyday applications, so we've been looking to expand this family of materials," said study lead author Kurt Lindquist, a Stanford doctoral student and Princeton University postdoctoral scholar in inorganic chemistry. "An unprecedented Au2+ perovskite could lead to intriguing new discoveries."
Gold's heavy electrons
Gold has traditionally been appreciated for its scarcity, malleability, and chemical inertness, allowing it to be easily moulded into jewellery and coins that do not tarnish. Gold's name suggests that no other metal has such a rich colour in its pure state, adding to its worth.
Karunadasa emphasised that gold's beauty and rarity stem from its physics.
The cause is relativistic effects, first proposed in Albert Einstein's theory of relativity. „Einstein showed us that objects get heavier when their velocity approaches a large percentage of the speed of light," Karunadasa remarked.
This applies to particles and has major implications for "massive" heavy elements like gold, whose atomic nuclei include many protons. These particles exert massive positive charge, causing negatively charged electrons to spin rapidly around the nucleus. Electrons become heavy and densely surround the nucleus, blunting its charge and allowing outside electrons to drift farther than in ordinary metals. Gold absorbs blue light and appears yellow due to electron energy rearrangement.
Due to relativity, gold naturally exists as Au1+ and Au3+, losing one or three electrons and rejecting Au2+. (The "2+" signifies a net positive charge from the loss of two negatively charged electrons, while the "Au" chemical symbol for gold comes from "aurum," Latin for gold.)
A vitamin C squeeze
Stanford researchers found that Au2+ can survive with the appropriate chemical arrangement. While researching magnetic semiconductors for electronic devices, Lindquist "stumbled upon" the new Au2+-harboring perovskite.
Lindquist mixed cesium chloride and Au3+-chloride in water with hydrochloric acid "with a little vitamin C thrown in," he stated. Vitamin C, an acid, contributes a negative electron to Au3+ to produce Au2+. Strangely, Au2+ is stable in solid perovskite but not solution.
Lindquist added, "In the lab, we can make this material using very simple ingredients in about five minutes at room temperature." "We get a dark green, nearly black powder that's heavy due to the gold."
Lindquist extensively tested the perovskite using spectroscopy and X-ray diffraction to determine its light absorption and crystal structure, believing they had found new chemical gold. Stanford research groups led by Young Lee, professor of applied physics and photon science, and Edward Solomon, Monroe E. Spaght Professor of Chemistry and photon science, studied Au2+'s behaviour.
The tests confirmed Au2+ in perovskite and added to Linus Pauling's century-long chemistry and physics career, which earned him the 1954 Nobel Prize in Chemistry and the 1962 Nobel Peace Prize. His early study involved gold perovskites with Au1+ and Au3+. Interestingly, Pauling later examined the structure of vitamin C, which is needed to make a stable perovskite with Au2+.
“We love Linus Pauling's connection to our work,” Karunadasa remarked. Perovskite synthesis is a good story."
Future research by Karunadasa, Lindquist, and collaborators will refine the new material's chemistry. Since electrons hop from Au2+ to Au3+ in an Au2+ perovskite, it may be used in magnetic and conductivity applications.
“We're excited to explore what an Au2+ perovskite could do,” Karunadasa said.
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