The influence of polyethylenimine molecular weight on hydrothermally-synthesised ZnO nanowire morphology

Abstract

Polyethylenimine (PEI) is a polymer commonly used in the hydrothermal synthesis of ZnO nanowires as an aspect-ratio enhancing agent. To this end, several studies have focused on optimising the PEI concentration to achieve the best results. However, the effect of the polymer's molecular weight on nanowire growth is relatively unstudied. We investigate three different molecular weights of PEI (Mw = 2000, 1300, and 800) at 8 mM, 6 mM, 4 mM and 2 mM concentrations. We find that the Mw = 2000 and Mw = 1300 varieties only yield homogeneous nanowires at 2 mM, while the Mw = 800 variety yields homogeneous nanowires over the entire range of concentrations investigated. The dimensions of the nanowires grown at 2 mM with PEI (Mw = 800, 1300) show minimal variation, with average diameters of 60 nm and average lengths of 2.5 µm, while the nanowires grown at 2 mM PEI (Mw = 2000) have an average diameter of 40 nm and an average length of 4 µm. The nanowires grown with PEI (Mw = 800) at concentrations of 8 mM, 6 mM, and 2 mM show a clear length dependency on concentration, with average lengths of 11 µm, 8 µm and 2.5 µm, respectively. A relationship between concentration and diameter is less clear, with average nanowire diameters of 90 nm, 40 nm and 60 nm, respectively.