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John O | April 2019

Nanowire-based thermochromic liquid darkens considerably when temperatures rise

By Josh Perry, Editor


Researchers from the Agency for Science, Technology, and Research (A*STAR) in Singapore developed a nanowire-based thermochromic liquid that darkens when heated and could replace organic dyes or liquid crystals in thermography systems.


The thermochromic liquid changing color in reaction to a change in temperature.
(A*STAR Institute of Materials Research and Engineering)


According to a report from A*STAR, the liquid overcomes previous challenges such as dyes degrading when exposed to light or liquid crystals requiring encapsulation to prevent degradation in air.


The liquid stemmed from research into semiconductor nanocrystals. Researchers noted that the crystals formed colloidal suspensions in certain solvents. In particular, researchers focused on colloidal antimony selenide (Sb2Se3) nanoparticles, which formed nanowires when heated and dissolved when cooled.


“Thanks to their broad light-absorbing behavior, a vial of Sb2Se3 nanowires formed by heating can appear very dark,” the report explained. “But a solution of their molecular precursors, which the nanowires revert to upon cooling, are relatively transparent.”


Scientists demonstrated that a solution of the molecular precursors was viable after two years in ambient conditions and its properties were retained after being heated and cooled hundreds of times. They also proved that the transition temperature could be tuned between 35-140°C by adding tin chloride to the mixture.


“When the researchers coated their thermochromic solution on to filter paper, they showed that it could differentiate between cooler and hotter regions of an irregularly heated surface,” the article added.


The research was recently published in Advanced Materials. The abstract stated:


“Liquid?based thermochromics can be incorporated into an arbitrarily shaped container and provide a visual map of the temperature changes within its volume. However, photochemical degradation, narrow temperature range of operation, and the need for stringent encapsulation processes are challenges that can limit their widespread use.


“Here, a unique solution?based thermochromic comprising ultrathin colloidal Sb2Se3nanowires in an amine–thiol mixture is introduced. The nanowires undergo reversible growth and dissolution with repeated cycles of heating and cooling between 20 and 160 °C, exhibiting intense and contrasting color changes during these processes.


“Furthermore, the transition temperature in which a change in color first appears can be continuously tuned over a range larger than 100 °C by introducing controlled amounts of Sn2+. The colloidal nanowire dispersion in the amine–thiol mixture retains its thermochromic properties over hundreds of temperature cycles, continuous heating at 80°C over months, and shelf life of up to 2 years in an open container under ambient conditions.


“To illustrate its utility as a robust liquid thermochromic, the nanowire solution is coated onto standard filter paper and its uses as a rewritable surface by thermal scribing, as well as an inexpensive means of visualizing the temperature distribution of an anisotropically heated block are demonstrated.”

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