Doping electrons to a semiconductor with hydrazine creates electrocatalyst

In the Matt Damon movie The Martian, Damon creates water from rocket fuel, hydrazine and hydrogen, nearly blowing himself up in the process. Now, researchers have discovered that turning this process on its head, by adding hydrazine to water, creates hydrogen. This changes conductivity in a semiconductor — a transformation with potential ramifications for electronics and the production of renewable fuels.

“We demonstrate in our study that a simple chemical treatment — in this case, a drop of dilute hydrazine (N₂H₄) in water — can dope electrons directly to a semiconductor, creating one of the best hydrogen-evolution electrocatalysts,” said Gautam Gupta, project leader at Los Alamos National Laboratory in the Light to Energy team of the lab’s Materials Synthesis and Integrated Devices group. The research was published in Nature Communications.

Understanding how to use a simple, room-temperature treatment to drastically change the properties of materials could lead to a revolution in renewable fuels production and electronic applications.

In recent years, the materials science community has grown more interested in the electrical and catalytic properties of layered transition metal dichalcogenides (TMDs). TMDs are primarily metal sulfides and selenides (eg, MoS₂) with a layered structure, similar to graphite; this layered structure allows for unique opportunities, and challenges, in modifying electrical properties and functionality.

Gupta and Aditya Mohite, a physicist with a doctorate in electrical engineering, have been pioneering work at Los Alamos seeking to understand the electrical properties of TMDs and use that knowledge to optimise these semiconductors for renewable fuels production.

In this work, MoS₂ shells — MoOx core nanowires, as well as pure MoS₂ particles and 2D sheets — are tested for electrocatalysis of the hydrogen evolution reaction. The addition of dilute hydrazine to MoS₂ significantly improves the electrocatalytic performance. Further characterisation shows that the MoS₂ changes from having semiconducting behaviour to having more metallic properties following the hydrazine exposure.

“The most interesting thing about this result is that it is different than conventional doping, where actual chemicals are added to a semiconductor to change its charge carrier concentration. In the case of hydrazine treatment, we are ‘doping’ electrons directly to the material, without modifying the original chemistry,” said Dustin Cummins, first author on this project, currently a postdoctoral researcher in the laboratory’s Sigma Division working on the DOE/NNSA CONVERT Program, exploring fuel fabrication for next-generation reactors.

Cummins first found the hydrogen-production result working with Gupta at Los Alamos as a graduate student research affiliate from the University of Louisville and he continued to conduct experiments and refine discussion while working as a postdoc.

“Hydrazine acting as an electron dopant in inorganic semiconductors has been observed since the 1970s, but there is limited understanding of the process,” Cummins said. “Our biggest hurdle was to prove that hydrazine was actually changing the conductivity of the MoS₂ system, and that is what results in increased catalytic activity.” This was demonstrated on single-flake devices, he said.

Multiple areas of Los Alamos staff expertise in layered semiconductors, chemistry, spectroscopy, electrical device fabrication and more all came together to provide some of the best understanding and mechanism to date for hydrazine acting as an electron dopant.

The paper not only presents one of the best hydrogen water splitting electrocatalysts to date, Gupta said, but also “opens up a whole new direction for research in electrochemistry and semiconductor device physics in general”.