New Conducting Metal-Organic Frameworks Get a Spark of Electricity
by Meg Marquardt
Materials Research Society | Published: 23 December 2013
Although they have only been around for just over a decade, metal-organic frameworks (MOFs) have already made a name as promising materials for gas storage, separation, drug delivery, and other conventional applications for porous materials. Many MOFs are inexpensive to synthesize, can be deposited under mild conditions, and are heat tolerant and chemically stable. But a new study, published in Science , takes these materials one step farther. By introducing a guest molecule within its pores, a well-studied MOF known as HKUST-1 becomes electrically conductive, opening an entirely new array of possible applications.
MOFs, as the name implies, are comprised of metal cations bonded to rigid organic "linkers," creating a stable nanoporous structure. With record-setting surface areas (as high as 7000 square meters per gram), they are ideal for applications such as CO 2 sequestration and catalysis. While efforts to develop MOFs for these applications made major strides over the past decade, electrically conducting frameworks were much more illusory. "What motivated us was the possible applications," says Alec Talin, materials scientist at Sandia National Laboratories, lead author of the study. He cites a host of promising avenues, such as novel electronic devices, photovoltaics, supercapacitors, and electro-catalysis.
But creating an electronically conductive MOF was not a simple matter. Until this innovation, virtually all known MOFs were electrically insulating, due to the primarily ionic nature of the metal-linker bonds. Rather than trying to change the framework itself, the Sandia team, in collaboration with the National Institute of Standards and Technology, took a different approach. "We decided to introduce a guest molecule into the pores," Talin says.
The molecule they chose, tetracyanoquinodimethane, or TCNQ, is also an insulating material. However, it was known that, in combination with other organic molecules or metal ions such as copper, TCNQ forms electron-conducting charge-transfer complexes. As such, the team started with a well-known copper-containing MOF known as HKUST-1. Talin soaked a thin film of HKUST-1 grown on patterned electrodes in a solution of TCNQ, causing the TCNQ to infiltrate the pores. The result was startling: the MOF film became >10 6 times more conductive than the film without TCNQ. "The framework is insulating, the guest molecule is insulating," says Talin, "but when they come together, they make a conducting material."
Moreover, the effect is tunable. By altering the exposure time to TCNQ, the magnitude of the conductivity could be controlled. TCNQ is the first step, says Talin; there are many other types of guest molecules that create other electronic behaviors leading to a wide range of applications. From here, Talin plans to investigate what happens when these MOF devices are scaled to much smaller dimensions, approaching molecular electronic devices that require a high level of adaptable control. Energy storage and energy conversion applications are also a top priority. "In chemistry, [MOFs} are one of the most exciting areas," says Talin, "and we're excited to take them to the next stage."
Read the abstract in Science here.