Introduction to Nanoscience and Nanotechnology. Chris Binns

in water, an applied magnetic field gradient will move all the particles through the water in the same direction to a point where they can be collected. On the laboratory scale, this is easy to demonstrate with a simple bar magnet as shown for the case of the removal of lead (Pb) contamination from water [28]. In this experiment, the nZVI nanoparticles were coated with a shell of graphene (see Chapter 4) to increase their effectiveness at adsorbing the Pb ions and the magnetic separation is demonstrated in Figure 2.17. The left image shows a suspension of the grapheme‐coated nZVI particles (labeled G‐nVZI) at a density of 1mg/ml and the right image demonstrates their separation with a bar magnet. Graphene alone is sometimes used as an adsorbent to filter Pb from water and it was found that the G‐nVZI particles are twice as effective in removing the contaminant as it is able to remove the Pb in both the neutral and ionic states. The ability to then separate the magnetic nanoparticles is a bonus.

      Magnetic separation of contaminants has only been demonstrated in the laboratory and is much more difficult in field applications. One issue is that it is difficult to apply sufficiently high field gradients over the distance scales required to produce magnetic separation on a workable timescale. In addition, if the particles are carried in a flow, the viscous drag will overcome the magnetic field gradients that can be applied in practice in even slow flows (see Chapter 8, Advanced Reading Box 8.1).

      

      2.7.2 Conversion of Waste Plastics to High‐Grade Materials (Upcycling)

Plastic is a polymer of small hydrocarbons like the CH₄ (ethylene) molecule shown in Figure 2.18a. The molecule has two spare bonds ready to combine with the spare bonds from other CH₄ molecules and heating ethylene gas at high pressure causes the molecules to form long chains (polymerize) of polyethylene (or polythene) as illustrated in Figure 2.18c. Modern plastics contain thousands to millions of molecules in the polymer chain. Polyethylene was first discovered accidentally in 1898 but was not produced commercially until 1939, though this was interrupted by the second world war when the process was kept secret. In the 1950s the introduction of catalysts enabled production at much lower temperatures and pressures producing a significant decrease in cost and the modern plastics industry emerged. Catalysts were briefly introduced in Chapter 1, Section 1.4 and their role is to provide a surface whose chemical properties reduce the energy required for a specific reaction between two other species to occur. Since it is only surface atoms that contribute, all catalysts are in the form of nanoparticles in order to maximize the surface area presented per gram of material.

Figure 2.18 Polymerization of ethylene to produce polyethylene. (a) Ethylene CH₄ molecule consisting of two carbon and four hydrogen atoms. (b) Bonding in CH₄ showing two spare bonds for linking to other CH₄ molecules. (c) Heating the ethylene gas at high pressure in the presence of a catalyst produces polyethylene.

      The properties of the solid polymer are highly controllable by changing the chain length, introducing other atoms and branching the polymer chains. This happens naturally to some extent and can be encouraged to form a low‐density light material (LDPE) or discouraged, which allows the linear chains to pack tightly and form a high‐density strong material (HDPE). This controllability coupled with the low cost of production has made plastic something of a wonder material that has become ubiquitous in modern society. Currently, around 400 million tons of plastic are produced every year by manufacturers and 75% of this goes into single‐use products.

The waste plastic has now become an environmental problem filling landfill sites and a significant proportion ending up permanently trapped in large scale circulating currents in the oceans known as gyres. The problem is amplified by the fact that plastic does not biodegrade and material that was produced half a century ago is still lingering as waste. Recycling plastic is thus a high priority but simple recycling, which consists of melting plastic products produces a lower grade, lower‐value product (termed downcycling) that is weaker than the original polymer. In practice, plastic can only be downcycled two or three times before it becomes unusable.

Figure 2.19 Catalyst for upscaling waste plastic. (a) Pt particles with a diameter of 2 nm on 100 nm SrTiO₃ cuboids used as a catalyst for upcycling polyethylene. The inset shows the size distribution of the Pt nanoparticles. (b) Single SrTiO₃ cuboid with its supported Pt nanoparticles. (c) Zoom in on red area in (b) showing the Pt nanoparticles in more detail. Also visible is the crystal lattice of the SrTiO₃.

      Source: Reproduced with the permission of the American Chemical Society from [29].

Recycling would be much more attractive if the waste plastic could be converted to higher‐value products (a process known as upcycling) such as lubricating oils and waxes that can be further processed into detergents and cosmetics. Converting waste plastics to other useful materials involves breaking carbon–carbon bonds to produce shorter chain molecules with a small mass distribution and this usually involves heating the plastic in the presence of hydrogen and a suitable catalyst, which is in the form of metal nanoparticles. Recently the catalyst shown in Figure 2.19, which consists of a combination of two nanostructures, was tested on a range of plastics [29]. The substrate consisted of 65 nm cuboids of strontium titanate (SrTiO₃), which is a highly crystalline material that has very good stability at high temperatures. Onto the cuboids was deposited 2 nm diameter platinum nanoparticles by a process known as chemical vapor deposition (CVD – see Chapter 5, Section 5.1.12). The catalyst was tested on a range of plastics ranging from high‐grade polyethylene to a single‐use plastic bag and in all cases was able to convert the complex branched high‐mass polymer molecules in the plastic to shorter linear molecules with a small mass range.

      Other recent innovations in upcycling have been to use waste plastic feedstock to produce carbon nanotubes and graphene [30, 31]. These materials are described in Chapters 3 and 4 and are destined to find applications in a range of emerging nanotechnologies described in those chapters. The processes to extract them from waste are described in Chapter 5, Section 5.1.12. Producing such a high‐value product from waste plastic is a further step toward encouraging recycling.

      This chapter has by far the widest scope in this book and each topic introduced here could easily occupy a book of its own. The treatment, therefore, has necessarily been superficial but a number of references are given for a more in‐depth study of various topics. The aim has been to give a flavor of the importance of nanoparticles in shaping our environment and also in addressing environmental issues. In the next chapter, we will bring our attention back to the research laboratory and discuss the fascinating world of carbon nanoparticles.

Problems

      1 1 In a volcanic eruption most of the mass of volcanic ash is distributed in