MS-1-P-6019 The influence of oxygen on the metal seeded growth of SnO₂ nanowires

Tin dioxide (SnO₂) is the most common sensor material for detection of reducing gases like CO, H₂, CH₄ etc. [1]. Many contributions about gas sensors based on SnO₂ nanowires have been published over the last decade, showing the great potential of nanostructured sensor materials [2]. One versatile method for the synthesis of nanowires is the well investigated vapor-liquid-solid (VLS) mechanism with gold particles as a catalyst [3]. The exact growth mechanism of metal oxide nanowires is however still a matter of discussion because of the insolubility of oxygen in gold.

In this work we describe the synthesis of SnO₂ nanowires by MOCVD technique and determine the influence of oxygen on the nanowire growth by methods of electron microscopy. The nanowires are grown via a reaction of TMT (Sn(CH₃)₄) with oxygen (O₂) on fused silica substrates with gold particles as a seed. Synthesis using optimized reaction parameters (t = 10 min, T = 800 °C, p = 1 Pa) and a TMT:O₂ molar ratio of 1:35 yields SnO₂ nanowires of 3 µm length and 30 nm width. The nanowires are terminated by a facetted particle with corresponding width (Fig. 1a). The growth direction was determined to be <101> of the cassiterite modification of SnO₂ from HRTEM (Fig. 1b-d). EDS measurements show that the particles are tin-free gold particles (Fig. 2).
If the TMT:O₂ ratio is raised to 1:1.3 by keeping the TMT flow rate constant and reducing the oxygen flow rate, the growth speed strongly increases by a factor of 15. The terminating particles are now of a round shape and about twice the diameter of the nanowires (Fig. 3). EDS measurements show significant amounts of tin in the gold particle, indicating the formation of a Sn-Au alloy.

The relationship between growth speed and TMT:O₂ ratio is supposed to be tied to the state of aggregation of the catalytic particle. A high TMT:O₂ ratio allows for accumulation of tin in the particle, leading to a liquefied particle and therefore higher surface diffusion rates to the particle-nanowire interface resulting in faster growth. A low TMT:O₂ ratio averts the accumulation of tin in the catalytic particle and liquefaction of the particle. This leads to lower diffusion rates of the reactants and thus slower growth of the nanowire.

These results clearly demonstrate that, in contrast to the classic VLS-mechanism, growth of SnO₂ nanowires can occur without liquefaction of the catalyst particle. Therefore the growth mechanism of metal oxide nanowires can be better described as a surface mediated "metal seeded growth" [4].

1. J. Watson, Sens. Actuators (1984), 5, 29-42.
2. B. Wang, J. Phys. Chem. C (2008), 12, 6643-6647.
3. R.S. Wagner, W.C. Ellis, Appl. Phys. Lett. (1964), 4, 89.
4. W. Mader, Cryst. Growth Des. (2013), 13, 572−580.