| AVS 57th International Symposium & Exhibition | |
| In Situ Microscopy and Spectroscopy Topical Conference | Tuesday Sessions |
| Session IS+SS-TuA |
| Session: | In Situ Microscopy/Spectroscopy – Interfacial Chemistry/Catalysis |
| Presenter: | K.M. McPeak, Drexel University |
| Authors: | K.M. McPeak, Drexel University M. Becker, University of Notre Dame B.A. Bunker, University of Notre Dame J.B. Baxter, Drexel University |
| Correspondent: | Click to Email |
Low temperature, aqueous solution synthesis is widely used to deposit ZnO nanowire arrays for applications such as dye sensitized solar cells. Zinc nitrate and hexamethylenetetramine (HMTA) are the most common precursors for the solution synthesis of ZnO nanowires; but after ten years of using this chemistry, the underlying mechanisms of the reaction remain unclear. We report on the in-situ x-ray absorption spectroscopy of ZnO nanowire growth from HMTA and zinc nitrate precursors. Time-resolved, in-situ x-ray absorption near-edge structure (XANES) spectra, at the Zn K-edge, give detailed information about the local structure of both Zn(II) in solution and the solid phases formed throughout the reaction. A principal component analysis (PCA) algorithm was employed to determine the number and type of probable species present during the growth of ZnO nanowires under real reaction conditions, with temperatures up to 90 ºC and concentrations ranging from 4 – 25 mM. Only two species were present for ZnO nanowire growth at all concentrations and temperatures studied: [Zn(6H2O)]2+ and ZnO(s). The proportions of these Zn species as a function of reaction time were determined quantitatively by least-squares fitting (LSF) the experimental time-dependent XANES spectra with linear combinations of the principal component spectra. PCA and target testing conclusively refute previous theories that ZnO nanowire growth from HMTA and zinc nitrate precursors occurs due to the thermal decomposition of an intermediate zinc-amine or zinc-HMTA complex. Additionally, XANES analysis shows that no zinc hydroxide intermediates exist above the detection limit of 0.05 mM. Beyond the focused investigation of ZnO, this study also establishes in-situ XANES spectroscopy, in combination with PCA and LSF, as an excellent quantitative tool to understand the solution synthesis of semiconductor nanostructures and thin films.