A New Approach of the Synthesis of SiO2 Nanowires By Using Bulk Copper Foils As Catalyst
Authors: Duconge José J , Morant Carmen , Gómez-Martínez Arancha , Márquez Francisco 
Published: February 2020

Silica nanowires (SiO2NWs) have attracted considerable attention over the last decades due to their diverse properties and broad range of applications. Among them, it is well-known the capacity of SiO2NWs to emit strong blue light. It is also widely recognized the versatility of this material as biosensor, since it can be easily functionalized in order to respond to several targets. Other applications include their potential use as anode electrode for lithium-ion batteries, or as protective layer against corrosion by exposure to the environment. In this regard, the use of a metal substrate is of paramount relevance; therefore, an adequate contact between the substrate and the nanostructures is required.
Especially numerous and varied are the procedures for the growth of SiO2NWs, being one of the most employed the so-called vapor-liquid-solid (VLS) method. By this mechanism, the nanowires grow from a metal nanoparticle when the adequate gaseous precursor is added. However, the main drawback of this mechanism resides in the need of a Si substrate for the synthesis of the SiO2NWs.
Apart from the original Au-Si system, a broad variety of other metals such as Fe, Co, Ni or Cu may act as catalysts for the growth. Particularly, of special interest is the case of copper, which is present in a large majority of electronic devices. Furthermore, the Cu–Si phase diagram shows a eutectic point at 802 ⁰C (16 wt% of silicon) promoting the formation of a liquid alloy at an accessible temperature for a relatively low concentration of silicon in copper.
This work presents a novel procedure for the growth of SiO2 nanowires directly from polycrystalline copper foils. The single-step synthesis procedure consists of a thermal treatment at 900 ◦C without the need for additional catalysts. As a result, nanowires with an average diameter of 100 nm are synthesized. A systematic study undertaken at different stages of the SiO2NWs growth confirmed the generation of nucleation centers on the Cu surface as well as revealed the existence of an intermediate gaseous SiO species at the synthesis temperature, which promotes the growth of the SiO2NWs via a VLS mechanism.
Figure 1 shows the dark field scanning transmission electron microscopy (DF-STEM) image of the as-synthesized SiO2NWs. As can be seen there, SiO2NWs growth is generated from substrate protrusions of around 200 nm in diameter, exceling from the Cu substrate. These protuberances produced during the thermal treatment could be the evidence of the formation of Cu-Si alloys required as initial step for generating the SiO2NWs.
Figure 2a shows the detail of a protrusion of the Cu substrate (in bright contrast) just under the nanowire (in dark contrast). EDX elemental maps performed on the inset area confirms the presence of Cu at the bottom of the nanowire (Fig 2b). Meanwhile, Figs 2c and 2d show a homogeneous distribution of Si and O across the whole nanowire.
This process with bulk Cu foils as catalytic agent allows the growth of SiO2NWs over metallic substrates, granting an adequate interconnection between both surfaces.

Figure 1