Researchers at North Carolina State University and Duke University have shown that a one-atom thick film of molybdenum sulfide (MoS2) may work as an effective catalyst for creating hydrogen. While not as efficient as platinum, the MoS2 think films are relatively inexpensive. The work, published in the ACS journal Nano Letters, thus opens the door for less expensive hydrogen production.
The researchers found that the thickness of the thin film is critical.
Here we leverage on the controlled growth of MoS2 atomically thin films to demonstrate that the catalytic activity of MoS2 for the hydrogen evolution reaction decreases by a factor of 4.47 for the addition of every one more layer. Similar layer dependence is also found in edge-riched MoS2 pyramid platelets. This layer-dependent electrocatalysis can be correlated to the hopping of electrons in the vertical direction of MoS2 layers over an interlayer potential barrier. Our experimental results suggest the potential barrier to be 0.119 V, consistent with theoretical calculations.
Different from the conventional wisdom, which states that the number of edge sites is important, our results suggest that increasing the hopping efficiency of electrons in the vertical direction is a key for the development of high-efficiency two-dimensional material catalysts.—Yu et al.
The effect of the thin films’ thickness came as a surprise to researchers, because it has long been thought that catalysis normally takes place along the edges of the material. Because thin films have very little ‘edge,’ conventional wisdom held that thin films were essentially catalytically inactive.
But the researchers discovered that a material’s thickness is important because the thinner the MoS2 thin film is, the more conductive it becomes—and the more conductive it becomes, the more effective it is as a catalyst.
The focus has been on creating catalysts with a large ‘edge’ side. Our work indicates that researchers may want to pay more attention to a catalyst’s conductivity.—Dr. Linyou Cao, assistant professor of materials science and engineering at NC State and senior author
Cao developed the technique for creating high-quality MoS2 thin films at the atomic scale in 2013. The current production of hydrogen from the atomically thin film is powered by electricity. His team is working to develop a solar-powered water-splitting device that uses the MoS2 thin films to create hydrogen.
Yifei Yu, Sheng-Yang Huang, Yanpeng Li, Stephan N. Steinmann, Weitao Yang, and Linyou Cao (2014) “Layer-Dependent Electrocatalysis of MoS2 for Hydrogen Evolution,” Nano Letters doi: 10.1021/nl403620g