## 3 Results and Discussion  
Gaseous and liquid carbon materials can readily be used to make HCSs in a one-stage CVD reactor [29; 34; 35]. However, the use of solid carbon materials requires the conversion of the solid material into gaseous products in a preliminary step prior to decomposition of the carbon source. Thus, the conversion of the solid plastic waste investigated in this study, that is, used polypropylene centrifuge tubes, required a two-stage CVD furnace for the synthesis of carbon materials. Polypropylene can readily be decomposed cleanly to propylene at T \(>\) 400 \({}^{\circ}\)C under inert gas conditions and thus a temperature of 500 \({}^{\circ}\)C was chosen for the first stage CVD furnace to make carbon spheres [26]. The temperature in the second stage of the reactor was initially chosen to range from 800 \({}^{\circ}\)C to 1000 \({}^{\circ}\)C to decompose the propylene. From this set of data, 900 \({}^{\circ}\)C was chosen for the study as this gave the SCSs a good yield and with diameters predominantly between 800 nm to 1200 nm (Supplementary Figure S2). The SEM images (Figure 2a) and TEM images (Figure 3a,b) of the SCSs, synthesized at 900 \({}^{\circ}\)C, show the expected morphology for the SCSs--a spherical shape with a smooth surface. The HCSs were made by a classical procedure [30; 31], but using PP as the carbon source. The propylene produced from the PP readily decomposed on the SiO\({}_{2}\) spheres at 900 \({}^{\circ}\)C. The initial silica spheres had a spherical shape and a size of ca. 1100 nm; after heating at 900 \({}^{\circ}\)C the spheres, as expected, shrank (ca. 1000 nm). The carbon was then passed over the SiO\({}_{2}\) template to give Si/C spheres. The SiO\({}_{2}\) template was then removed by HF to give the HCSs. SEM and TEM images confirmed that the HCSs were hollow and had a diameter close to 1000 nm and a shell width of ca. 35 nm. (Supplementary Figure S2; Figure 2b, and Figure 3c,d). The SEM image showed that some of the HCSs had holes in the surface due to insufficient coverage of the template by the carbon. Longer reaction times and higher temperatures gave thicker shells. The NHCSs materials were made by mixing melamine mixed with the solid plastic waste and by then heating the mixture [36]. The melting point of melamine (345 \({}^{\circ}\)C) [37] is similar to the _decomposition_ temperature of PP (which melts at 130 \({}^{\circ}\)C). The removal of the SiO\({}_{2}\) template produced NHCSs material with a similar diameter (1000 nm) and a slightly thicker shell (40 nm) when compared to the HCSs material (Supplementary Figure S2; Figure 3e,f). This was due to the extra amount of reactants that were passed over the template (i.e., 1.0 g melamine and ca. 2 g PP), and was also mirrored by the lack of broken spheres seen in the SEM images of the NHCSs (Figure 2c). High magnification images  
Figure 2: SEM images of the (**a**) SCSs, (**b**) HCSs and (**c**) NHCSs (900 \({}^{\circ}\)C, 1 h reaction). indicated that the HCSs and NHCSs sphere surfaces were rough (Figure 4). This was unexpected as most studies have shown that the surface of hollow carbons is relatively smooth [30; 31]. The surface areas were measured and the SCSs had a low surface area as expected for CVD synthesized SCSs (4 m\({}^{2}\)g\({}^{-1}\)), and these values increased to 25.7 m\({}^{2}\)g\({}^{-1}\) (HCSs) and 42.1 m\({}^{2}\)g\({}^{-1}\) (NHCSs) for the templated materials. These values are low, implying poor porosity. This will most likely have implications for the reactivity of the shells if these spheres are used in stabilizing metal catalysts on their carbon surfaces. TGA data were recorded for the PP and PP silica coated materials (not shown), and the silica residues determined from the carbon oxidation reaction in the TGA provided for a measure of the  
Figure 4: High magnification TEM images of the surface of (**a**) HCSs and (**b**) NHCSs. Figure 3: SEM images of (**a,b**) SCSs, (**c,d**) HCSs and (**e,f**) NHCSs (900 \({}^{\circ}\)C, 1 h reaction). carbon yields (ca.