Science for our
Nation's
Energy Future

Energy Frontier Research Center

Community Website
Frontiers in
Energy Research
Newsletter
April 2012

Form Gives Function in Ultrathin Crystals

Atomically thin sheets of molybdenum sulfide show enhanced light emission and catalytic activity

Bryce Sadtler

(A) Model of the layered crystal structure of MoS2 with molybdenum atoms in blue and sulfur atoms in yellow. (B) Schematic of a core-shell MoO3-MoS2 nanowire array for electrocatalytic hydrogen production.

Molybdenum sulfide or MoS2 is a candidate material for use in solar energy conversion and fuel production. Scientists within two Energy Frontier Research Centers have demonstrated they can optimize the properties of MoS2 by structuring it into atomically thin sheets. Researchers at the Redefining Photovoltaic Efficiency through Molecular Scale Control Center found that single layers of MoS2 display dramatically enhanced light emission. At the Center on Nanostructuring for Efficient Energy Conversion, thin layers of MoS2 grown to form a shell over the surface of molybdenum oxide nanowires efficiently catalyze the production of hydrogen from water.

Single Layers of MoS2 Become Luminescent

The arrangement of atoms in a solid is intimately linked to the physical and chemical properties it exhibits. Crystals of MoS2 possess a sheet-like structure in which each layer consists of a plane of molybdenum atoms sandwiched between two planes of sulfur atoms. The sheet-like structure shapes the energy levels that electrons occupy within the MoS2 crystal. In turn, the electronic configuration determines the way the material absorbs and emits light. MoS2 is classified as a semiconductor material because there is an energy gap between the valence band of filled electronic states and the conduction band of empty states. Semiconductors are the key component in solar cells, where light with an energy greater than the band gap can be used to promote electrons from valence to conduction band to produce electricity. Solar cells operate most efficiently at light energies slightly greater than the band gap. However, the specific electronic configuration of bulk MoS2 crystals make it a poor absorber of light near the band gap energy.

Researchers in the Redefining Photovoltaic Efficiency through Molecular Scale Control Center mechanically exfoliated sheets of MoS2 to obtain ultrathin crystals ranging from 1 to 6 atomic layers. They found that the electronic energy levels governing the way that MoS2 absorbs and emits light were sensitive to the number of atomic layers in the ultrathin MoS2 crystals. A crossover in the lowest-lying electronic level of the conduction band in MoS2 dramatically changes the optical properties of a single layer of MoS2 compared to multiple layers. An increase in light emission efficiency by over two orders of magnitude indicates that direct light absorption in single-layer MoS2 is much more efficient compared to that in multiple layers.

“The strong luminescence of the single MoS2 layers may make it useful as a nanoscale sensor, and the ability to tune the absorption and emission of light in ultrathin MoS2 can be applied to novel photovoltaic designs,” said Tony Heinz, who led the project at Columbia University.

High Surface Area Increases Catalytic Activity

The edge-site atoms around the periphery of the layers in MoS2 crystals are much more active at catalyzing the production of hydrogen from water compared to atoms within the crystal’s bulk. The electrochemical reduction of water to produce hydrogen gas could provide a route to storing solar energy in the form of a chemical fuel. For MoS2 to be an efficient hydrogen reduction catalyst it must expose a large number of the active edge-site atoms at the crystal surface. Scientists at the Center on Nanostructuring for Efficient Energy Conversion are trying increase the catalytic activity of MoS2 by growing ultrathin sheets of MoS2 over a nanostructured template to make a catalyst with a high surface area. The template consists of an array of molybendum oxide, or MoO3, nanowires with dimensions that are approximately 1000 times smaller than the diameter of human hair.

“Nanostructuring MoS2 allows you to massively increase its surface area for hydrogen production,” said Zhebo Chen, a graduate student in the Center, “but for such nanostructures to work, one must also develop an architecture that allows for electric charge to be transported efficiently.”

Collaborators at the University of Louisville grew an array of molybdenum oxide, or MoO3, nanowires that are electrically conductive to use as a scaffold for growing high-surface-area MoS2. In the laboratory of Thomas F. Jaramillo at Stanford University, Chen then chemically converted the surface of the nanowires through a sulfidation reaction to form a thin conformal layer of MoS2 around the MoO3 nanowires, producing a core-shell structure. The thickness of the MoS2 shell can be tuned by the temperature of the sulfidation reaction. Composite MoO3-MoS2 core-shell nanowires fabricated at 200°C efficiently produced hydrogen with relatively low energy input. Importantly, the MoS2 shell also makes the nanowires stable in the acidic environment typically used in the electrocatalytic production of hydrogen. Future work includes improvement of the catalytic activity of the nanowires by optimizing the number of exposed edge sites of the MoS2 shells.

More Information

Chen Z, D Cummins, BN Reinecke, E Clark, MK Sunkara and TF Jaramillo. 2011. “Core-shell MoO3-MoS2 Nanowires for Hydrogen Evolution: A Functional Design for Electrocatalytic Materials.” Nano Letters 11(10):4168-4175. DOI:10.1021/nl2020476

Mak KF, C Lee, J Hone, J Shan and TF Heinz. 2010. “Atomically Thin MoS2: A New Direct-Gap Semiconductor.”  Physical Review Letters 105(13):136805-1136805-4. DOI: 10.1103/PhysRevLett.105.136805

Acknowledgments

The preparation and optical characterization of atomically thin MoS2 was supported under the Redefining Photovoltaic Efficiency through Molecular Scale Control Energy Frontier Research Center funded by DOE, Office of Science, Office of Basic Energy Sciences. Transport measurements and modeling performed at Case Western University were supported by the National Science Foundation.

Physical and electrochemical characterization of the nanowires was supported as part of the Center on Nanostructuring for Efficient Energy Conversion, an Energy Frontier Research Center, funded by DOE, Office of Science, Office of Basic Energy Sciences. Early development of the nanowire synthesis was funded by the DOE, Office of Energy Efficiency and Renewable Energy. Growth of MoO3 nanowires was supported under the DOE Experimental Program to Stimulate Competitive Research.

About the author(s):

  • Bryce Sadtler is a Beckman Postdoctoral Scholar at the California Institute of Technology and a member of the Light-Material Interactions in Energy Conversion, an Energy Frontier Research Center. His research interests are in light-driven processes for directing the morphology of inorganic structures and the design of nanoscale materials for energy conversion and storage.

The Magic of Molybdenum

Thin layers could help capture more solar energy and turn it into fuel

(A) Model of the layered crystal structure of MoS2 with molybdenum atoms in blue and sulfur atoms in yellow. (B) Schematic of a core-shell MoO3-MoS2 nanowire array for electrocatalytic hydrogen production.

Would you buy a car that only worked when the sun was shining? To make intermittent solar power a major part of the nation’s energy landscape means harvesting light efficiently and turning that light into fuel. New materials are needed for these tasks. Research by two Energy Frontier Research Centers showed that thin layers of molybdenum sulfide crystals could address both issues. They found that single atomic layers of molybdenum sulfide absorb and emit light more efficiently than multiple layers of the material. Scientists also wrapped tiny wires in molybdenum sulfide sheets, creating catalysts that efficiently produced hydrogen fuel from water. This work is part of the ongoing national effort to design and discover materials that will make alternative power part of the mainstream. The Redefining Photovoltaic Efficiency through Molecular Scale Control Center, led by Columbia University, did the light-harvesting work. The Center on Nanostructuring for Efficient Energy Conversion, led by Stanford University, did the hydrogen catalysis work.

Written by Bryce Sadtler and Kristin Manke

More Information

Chen Z, D Cummins, BN Reinecke, E Clark, MK Sunkara and TF Jaramillo. 2011. “Core-shell MoO3-MoS2 Nanowires for Hydrogen Evolution: A Functional Design for Electrocatalytic Materials.” Nano Letters 11(10):4168-4175. DOI:10.1021/nl2020476

Mak KF, C Lee, J Hone, J Shan and TF Heinz. 2010. “Atomically Thin MoS2: A New Direct-Gap Semiconductor.”  Physical Review Letters 105(13):136805-1136805-4. DOI: 10.1103/PhysRevLett.105.136805

Disclaimer: The opinions in this newsletter are those of the individual authors and do not represent the views or position of the Department of Energy.