Transport Limited Growth of Zinc Oxide Nanowires

CRYSTAL GROWTH & DESIGN

Transport Limited Growth of Zinc Oxide Nanowires

2009 VOL. 9, NO. 6 2783–2789

Janice E. Boercker, Jillian B. Schmidt, and Eray S. Aydil* Department of Chemical Engineering and Materials Science, UniVersity of Minnesota, 421 Washington AVenue SE, Minneapolis, Minnesota 55455 ReceiVed January 9, 2009; ReVised Manuscript ReceiVed March 27, 2009

ABSTRACT: Vertical arrays of crystalline zinc oxide (ZnO) nanowires grown on various substrates find applications in dyesensitized and hybrid organic/inorganic bulk-heterojunction solar cells. The ability to grow dense nanowires at high rates and the fundamental understanding of the growth process are important for these applications. Herein, we show that heterogeneous growth of ZnO nanowires on substrates seeded with ZnO nanoparticles in an aqueous solution of methenamine and zinc nitrate is mass transport limited. Mass transport limited growth leads to an inverse relationship between the nanowire dimensions (height and diameter) and the nanowire number density. This mass transport limitation also leads to nonuniform growth near the boundaries between seeded and unseeded regions. Stirring the reaction solution increases the nanowire growth rate. Experimental results were interpreted within the framework of two simple but nontrivial models of the solution phase species transport and the nanowire growth. Additionally, it was determined that the anisotropic growth is due to the intrinsic growth kinetics of the (101j0) and (0001) surfaces of ZnO in zinc nitrate and methenamine and not due to the growth process being mass transport limited as previously suggested. Introduction Zinc oxide (ZnO) nanowires grown from aqueous solutions of zinc nitrate and methenamine on transparent conducting substrates preseeded with ZnO nanoparticles have been used as the photoanode in dye-sensitized solar cells1-3 and in hybrid organic/inorganic bulk-heterojunction photovoltaic devices.4 For thick photoanodes (e.g., >10 µm), the nanowire morphology can potentially provide higher charge collection efficiencies than nanoparticles,5,6 but the overall power conversion efficiency of nanowire-based dye-sensitized solar cells is still an order of magnitude lower than that achieved with typical TiO2 nanoparticle dye-sensitized solar cells.7 One of the reasons for the lower efficiency is the significantly lower surface area of the ZnO nanowires compared to the TiO2 nanoparticle films.2 One approach to increasing the nanowire surface area is to grow taller nanowires. However, the extraordinarily slow growth rate of the nanowires (