# Dry Sintered Metal Coating of Halloysite Nanotubes
## 3 Results and Discussion
The nickel-coated tubes (d) can also be seen to maintain the individual tubes as with the barium. However, the nickel does not form as thick of a sheath as the barium. This is attributed to the smaller size of the nickel ions as compared to the barium, allowing the nickel to bond in the nanotube pores. Figure 3: SEM of (**a**) raw HNTs, (**b**) barium sintered HNTs, (**c**) iron sintered HNTs, and (**d**) nickel sintered HNTs. To verify the metallic coatings of the nanotubes, EDS (Energy Dispersive X-ray Spectroscopy) analysis was performed to test for the elements present in the sample (Figure 4). The raw halloysite (a) shows the aluminum, silicon, and oxygen expected of the aluminosilicate nanotubes. The subsequent three (b-d) show definitive peaks for the elements sintered, ensuring their presence in the samples. Multiple EDS characterizations were performed, and all peaks found were similar to the ones presented in Figure 4. Slight changes were noted due to the relative nature of the sample and the small sample window of the EDS; however, the peaks present during the analysis confirmed the presence of the suspected metal dopant. We have also been successful in sintering copper and lithium onto the surface of the HNTs; copper for its potential as a conductive sheath for micro-fabricated antennas and theranostic applications, and lithium for its common use in neutron detection or time-release coatings for bipolar treatments. EDS data for the lithium are inconclusive, due to the light weight of the element and the use of a beryllium lens for the system that filters out the respective signal. However, copper did show definitive elemental mapping. Imaging revealed that copper acted as nickel did, and this is attributed to the 2+ ion involved in that reaction. Images of the lithium proved to be cloudier than raw halloysite, indicating that deposition did occur onto the external surface of the nanotubes. In this study, dry sintering of metal acetylacetonates mixed with halloysite was shown as a viable method for coating HNTs in the desired metal due to the HNTs' inherent net negative surface charge. The metal acetylacetonate coating was confirmed via SEM, TEM, and EDS, where visual and elemental analysis showed concrete deposition of the material. Metal nanoparticles (NPs), such as silver NPs, copper NPs, gold NPs, and others, can be directly deposited onto the surfaces of HNTs. Over the last five years, a number of methods have been developed for the metallization of HNTs. For example, [PERSON] et al. [27] and [PERSON] and [PERSON] [28] describe processes used to bind gold compounds to the surface of halloysite. Methods that load HNTs with copper [29; 30] and  
Figure 4: EDS of (**a**) raw HNTs, (**b**) barium sintered HNTs, (**c**) iron sintered HNTs, and (**d**) nickel sintered HNTs. silver [3] for use as an antibacterial agent have also been recently developed. All these fabrication methods include lengthy and multi-step processes that include metal-salts or organic compounds, complexing and reducing agents, high temperatures, and expensive equipment to achieve metallization of HNT surfaces. Our method has the advantages of lower cost, simpler fabrication conditions, low energy consumption, environmental protection, and does not require the use of caustic or toxic compounds. Halloysite has been extensively studied for the reinforcement of medical grade polymer matrices [17; 31; 32] and in the sustained release of anti-corrosion agents [33], antibiotics [32; 34; 35], growth factors [15], and chemotherapeutic compounds [17]. Halloysite nanotube-supported metal nanoparticles have potential uses in biomedical devices, antimicrobial surfaces, theranostic delivery systems, radiation absorptive composites, plastic/elastomer composites, electronic components, and as industrial catalysts. As an example, the addition of the contrast agent to halloysite rather than the polymer matrix could potentially provide greater mechanical enhancement rather than barium added directly to the polymer.