# Dry Sintered Metal Coating of Halloysite Nanotubes
## 3 Results and Discussion  
Metal acetylacetonates provide a means of generating positive metal ions through thermal degradation of the acetylacetone bond that interact readily with the native negative charges on the outer halloysite surface, allowing for coating of the metals without the use of wet chemical processes, flowed gas, or vacuum (Figure 1). Furthermore, the decay products are non-toxic, negating the need for filtration of the out gassed products during fabrication and allowing for a much easier path to a full-scale manufacturing operation without the need for environmental permits [21]. The metals chosen for this process were among many that had readily-available acetylacetonate compounds that could be used, but all have use in important applications. Barium acetylacetonate was selected, as barium sulfates are commonly used as a contrast agent in biomedical applications due to the strong mass attenuation coefficient of barium at the energies of medical grade photons. Additionally, barium titanate nanowires have been used as mechanical components in accelerometers due to their piezoelectric nature [22]. New fabrication techniques in halloysite could generate a host of new microelectromechanical (MEMS) devices. Iron acetylacetonate was chosen due to the potential magnetic qualities that could be achieved. The traditional layer-by-layer approach typically used in the creation of magnetic sheaths for the nanotubes provide very thin layers and typically use cobalt or nickel for the magnetic material [23; 24]. Nickel has many potential uses as a catalyst, and as such, nickel acetylacetonate was used to obtain nickel coatings on the tubes for a potentially greater surface area. Nickel and iron also show promise as battery materials when nanostructured [25; 26]. Raw HNTs are transparent to the TEM, consisting of low-density thin-walled materials. With the introduction of heavier metals, the transparency is overcome. Shown in Figure 2 are transmission electron microscope (TEM) images taken of the resulting metal-coated halloysite after the above-described process. Here, the differing charges of the metal ions dictate the characteristics of the metal coating during the sintering process. For the barium and nickel (B,D), tight bonding to the external lumen of the halloysite is observed (Figure 2). Figure 1: Chemical bonding structures for metal acetylacetonates (**a**) and HNTs (**b**). Figure 2: TEM of (**A**) raw HNTs, (**B**) barium sintered HNTs, (**C**) iron sintered HNTs, and (**D**) nickel sintered HNTs. This is likely due to the M\({}^{2+}\)-charge state of these metal ions, allowing for both bonds to be linked to the negatively-charged oxygen found on the surface of the halloysite. In contrast, the iron (C) (a Fe\({}^{3+}\) ion) is seen to clump rather than evenly disperse along the length of the nanotube. This is due to the extra bond on the ion that is unable to bond fully onto the exterior lumen. Two of the available bonds oxidize onto the surface of the halloysite, while the remaining bond oxidizes, as the sample was not done under vacuum. This leads to another 1-oxygen bond on the surface of the iron, which attracts another iron ion, leading to two free 1-oxygen bonds. As the iron continues to oxide together, the clumps seen in Figure 2B are formed. In considering larger bulk samples of the halloysite, scanning electron microscope (SEM) images were taken and are shown in Figure 3. The raw halloysite (a) exhibits a lighter nature, and can be seen to be distinct tubules; however, as the metals are applied, there is a contrast in the samples correlated directly to the increased density of the material. This allows visual confirmation of the coating material to be achieved as the metals (barium, iron, nickel) are significantly denser than the aluminum, silicon, and oxygen composing the raw halloysite. The barium-coated tubes (b) have tight sheaths, but can still be seen as individual tubes. The iron-coated tubes (c) can be seen to aggregate with individual tubes on top of the sample.