Oregon State University pigment researchers are using a rare mineral discovered in Norway more than a century ago as a road map for creating new yellows, oranges and reds that are vibrant, durable, non-toxic and inexpensive.
The new pigments also carry energy-saving potential: Their ability to reflect heat from the sun means that buildings and vehicles coated in them will require less air conditioning.
The study led by Mas Subramanian, who made color history in 2009 with the discovery of a vivid blue pigment now known commercially as YInMn Blue, was published in Chemistry of Materials.
Funded by the National Science Foundation, the work centers around the crystal structure of thortveitite, a silicate containing scandium and yttrium. A silicate is any compound featuring silicon and oxygen.
Thortveitite isn't known for vibrant colors, but by introducing the abundant elements nickel, zinc and vanadium into a thortveitite-like crystal lattice, scientists have produced a collection of intense yellow, orange and reddish pigments.
"The resulting color depends on the concentration and structural environment of divalent nickel, which is the primary chromophore responsible for the color," said Subramanian, University Distinguished Professor of Chemistry and the Milton Harris Professor of Materials Science in the OSU College of Science.
Chromophores are the parts of a molecule that determine color by reflecting some wavelengths of light while absorbing others.
"Although divalent nickel is known to produce yellow and green colors in inorganic compounds, it rarely produces oranges and/or reds," Subramanian said. "The discovered pigments are stable under high temperatures and in acidic environments with no change in the structure or color properties, and they can be made in air at relatively low temperatures, around 750 degrees Celsius, which makes large-scale production feasible."
Subramanian notes that the search for vivid inorganic pigments has been going on throughout recorded history, often with limited success due to stability and toxicity problems.
"Because of that, currently used inorganic yellow, orange and red pigments are being phased out for use in large-scale coatings and color applications," he said. "A few replacements have been suggested but they all have stability issues and most are not commercially viable."
In 2009, Subramanian made a chance discovery that rocked the color world when, while experimenting with new materials for electronics applications, he came upon a deep, bright, durable blue. It was the first new inorganic blue to be discovered in more than two centuries. Among its other achievements, YInMn Blue - named for components yttrium, indium and manganese - inspired a new Crayola crayon color: Bluetiful.
"Most pigments are discovered by chance," Subramanian said. "The reason is because the origin of the color of a material depends not only on the chemical composition, but also on the intricate arrangement of atoms in the crystal structure. So, someone must make the material first in a laboratory, then study its crystal structure thoroughly to explain the color."
Since discovering YInMn Blue, Subramanian has been working on a rational design approach to take some of the luck out of the pigment search process. However, despite recent advancements in quantum mechanical theories and computational methods, predicting a crystal structure that will produce an intense inorganic pigment of a desired color is still tricky.
"Serendipity will still play a role in new pigment discoveries as science doesn't always follow a prescribed path, and that adds to the excitement of doing color research," he said. "But our approach holds the promise of accelerating the development of new pigments with targeted colors and functionalities. More and more, we're gaining an understanding of the complex interplay between chemical composition, crystal structure and light interaction to precisely control the absorption and reflection of light across the visible spectrum."
Collaborating with Subramanian were OSU's Yi-Chia Lin, Anjali Verma and Jun Li, and Arthur Ramirez of the University of California, Santa Cruz.
