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Center for Nonlinear Studies |
Heterogeneous catalysis is a crucial component of several key modern industrial processes including petrochemical cracking, catalytic converters in automobiles, fertilizer production, and pharmaceutical manufacturing. Catalysts are constantly being re-engineered to improve selectivity, increase the inherent efficiency, or create new materials. Despite the great importance of catalysts for the modern chemical/materials industries, most discoveries have occurred by brute force testing rather than an understanding of catalytic processes. A better understanding of catalysis at the nanoscale could guide catalyst selection/engineering, possibly leading to more rapid advancements in chemical/materials processing.
With new microscopy techniques such as environmental transmission electron microscopy (ETEM), it is possible to directly study heterogeneous catalysts in real time with lattice resolution to motivate the development of new catalysis models/theories. Group IV nanowires grown by vapor-liquid-solid (VLS) method are an ideal model system to study heterogeneous catalysis and guide the selection of novel nanowire catalysts/reactor conditions through a fundamental understanding of growth. In addition to being a model system, catalytically grown nanowires may have applications in electronic devices [1], photonics [1], and energy storage [2]. Studies performed on Si whiskers in the 1960s [3] established the general framework of catalytic nanowire growth (Fig. 1A), explaining the VLS mechanism through the equilibrium phase diagram. However, the equilibrium VLS model cannot account for several key aspects of growth including: the formation of kinetically stabilized liquids below the eutectic temperature [4], nucleation of metastable catalyst (Fig. 1B) [5]/wire twin defects [6], dynamics of Ge-Si heterojunction interface formation [7], and roughening at the triple phase boundary (Fig. 1C) [8].
These phenomena were modeled/rationalized using computational thermodynamics, kinetic nucleation models, and quasi-static surface tension/energetics of the catalyst/wire/vapor triple phase boundary. It was determined that under many reactor conditions nanowire catalysis is primarily driven by kinetic nucleation barriers and surface energetics, which allow for large supersaturations opening kinetic pathways unavailable to bulk Au-Ge and Au-Si alloys.
[1] C. M. Lieber, and Z. L. Wang, MRS Bull. 32 99 (2007)
[2] C. K. Chan et al., Nat. Nano. 3, 31 (2008)
[3] R. S. Wagner, and W. C. Ellis, Appl. Phys. Lett. 4, 89 (1964)
[4] A. D. Gamalski et al., Nano Lett. 10, 2972 (2010)
[5] A. D. Gamalski et al., Phys. Rev. Lett. 108, 255702 (2012)
[6] A. D. Gamalski et al., Nano Lett. 14, 1288 (2014)
[7] A. D. Gamalski et al. ACS Nano, 7, 7689 (2013)
[8] A. D. Gamalski et al. J. Phys. Chem. C 115, 4413 (2011)
Host: Neil Henson