Sputtering

Sputtering in progress. [Some rights reserved by Engenharia de Superficies, licensed under CC BY 2.0]

Sputtering is one of the simplest methods of coating nanofibers with an inorganic layer. A wide variety of inorganic material may be sputtered on nanofibers with gold and platinum routinely sputtered on nanofiber membrane to improve its conductivity prior to visualization under the scanning electron microscope (SEM). There are many other commercially available target materials that can be used for sputtering such as Ti, ZnO, ITO, Cr and Mo to name a few.

The surface topography of electrospun fibers has been shown to be further exaggerated with sputtering power. Wang et al electrospun poly(methyl methacrylate) (PMMA)/Organically Modified Montmorillonite (O-MMT) composite fibers that showed a coarse surface topography attributed to phase separation. Sputtering was carried out using Ti target for 90 minutes at power of 80 w, 100 w, 120 w and 140 w. With increasing sputter power, the surface of the electrospun fibers starts to exhibit more pronounced dimples as the TiO₂ impacts the fiber. At 140 w, cavities can be seen on parts of the microfibers as the high energy TiO₂ nanoparticles shoot through it. The catalytic performance of the functionalized fiber improves with increasing fiber surface porosity. However, it starts to drop at the highest power due to degradation and removal of fiber material.

Despite the ease of this process to coat the nanofiber membrane, the limitation is that it forms a thin layer of the inorganic material at the top, exposed surface of the nanofibers. Nanofibers that are shielded from the layers above and fibers at the underside will not be coated using this process. However, in some applications, such non-uniform distribution of the coated layer is preferred. Wu et al (2013) used sputtering to construct conductive nanotroughs with electrospun nanofibers as a template material. Having the top surface of the nanofibers coated and the bottom uncoated enables them to transfer the coated nanofibers on to a silicon substrate using dropcast. The nanofibers templates were subsequently removed so that the coating formed nanotrough on the substrate. This design was shown to demonstrate superior transparency, flexibility and conductivity. This process is also limited to inorganic target materials and needs to be carried out in an inert gas condition.

Schematic of nanotroughs on substrate using sputtering to coat on nanofibers template.

Table 1. Nanofiber membrane sputter coated.

Polymer	Coating	Reference
Polyamide-6	TiO2	Wei et al 2006
Polyamide-6	Tin-doped indium oxide (ITO)	Wei et al 2010
Polyamide-6	Fe2O3	Cai et al 2008

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