Hematene Joins Parade of New 2D Materials

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Rice materials scientist Pulickel Ajayan and colleagues extracted 3-atom-thick hematene from common iron ore. Hematene may be an efficient photocatalyst, especially for splitting water into hydrogen and oxygen, and could also serve as an ultrathin magnetic material for spintronic-based devices, the researchers said.

“2D magnetism is becoming a very exciting field with recent advances in synthesizing such materials, but the synthesis techniques are complex and the materials’ stability is limited,” Ajayan said. “Here, we have a simple, scalable method, and the hematene structure should be environmentally stable.”

Ajayan’s lab worked with researchers at the University of Houston and in India, Brazil, Germany and elsewhere to exfoliate the material from naturally occurring hematite using a combination of sonication, centrifugation and vacuum-assisted filtration.

Hematite was already known to have photocatalytic properties, but they are not good enough to be useful, the researchers said.

“For a material to be an efficient photocatalyst, it should absorb the visible part of sunlight, generate electrical charges and transport them to the surface of the material to carry out the desired reaction,” said Oomman Varghese, a co-author and associate professor of physics at the University of Houston.

“Hematite absorbs sunlight from ultraviolet to the yellow-orange region, but the charges produced are very short-lived. As a result, they become extinct before they reach the surface,” he said.

Hematene photocatalysis is more efficient because photons generate negative and positive charges within a few atoms of the surface, the researchers said. By pairing the new material with titanium dioxide nanotube arrays, which provide an easy pathway for electrons to leave the hematene, the scientists found they could allow more visible light to be absorbed.

The researchers also discovered that hematene’s magnetic properties differ from those of hematite. While native hematite is antiferromagnetic, tests showed that hematene is ferromagnetic, like a common magnet. In ferromagnets, atoms’ magnetic moments point in the same direction. In antiferromagnets, the moments in adjacent atoms alternate.

Unlike carbon and its 2D form, graphene, hematite is a non-van der Waals material, meaning it’s held together by 3D bonding networks rather than non-chemical and comparatively weaker atomic van der Waals interactions.

“Most 2D materials to date have been derived from bulk counterparts that are layered in nature and generally known as van der Waals solids,” said co-author Professor Anantharaman Malie Madom Ramaswamy Iyer of the Cochin University of Science and Technology, India. “2D materials from bulk precursors having (non-van der Waals) 3D bonding networks are rare, and in this context hematene assumes great significance.”

According to co-author Chandra Sekhar Tiwary, a former postdoctoral researcher at Rice and now an assistant professor at the Indian Institute of Technology, Gandhinagar, the collaborators are exploring other non-van der Waals materials for their 2D potential.

Iyer and Rice visiting student Aravind Puthirath Balan of Cochin University are lead authors of the study. Co-authors are Sruthi Radhakrishnan, Amey Apte, Carlos de los Reyes, Vidya Kochat, Robert Vajtai, Angel Martí and Gelu Costin of Rice; Cristiano Woellner and Douglas Galvao of the State University of Campinas, Brazil; Shyam Sinha and Peter van Aken of the Max Planck Institute for Solid State Research, Stuttgart, Germany; Liangzi Deng, Banki Manmadha Rao, Maggie Paulose and Ram Neupane of the University of Houston, Ching-Wu Chu of the University of Houston and Lawrence Berkeley National Laboratory, Berkeley, Calif.; and Avetik Harutyunyan of the Honda Research Institute USA Inc., Columbus, Ohio.

Ajayan is chair of Rice’s Department of Materials Science and NanoEngineering, the Benjamin M. and Mary Greenwood Anderson Professor in Engineering and a professor of chemistry.

The research was supported by India’s Ministry of Human Resource Development, the U.S.Army Research Office Multidisciplinary Research Institute, the Air Force Office of Scientific Research, the São Paulo Research Foundation, the T.L.L. Temple Foundation, the John J. and Rebecca Moores Endowment, the State of Texas, Shell International Exploration and Production Inc. and the Neutrino Observatory.