Green Nanomaterials. Siddharth Patwardhan

      Image courtesy of Lightspring/Shutterstock.

      We have learnt the benefits of green synthesis, but before we can explore sustainable bioinspired green synthesis of nanomaterials, it is important to understand what nanomaterials are, and how we can use them. Nanomaterials, nanotechnology and particularly biomedical applications of nanotechnology have captured our imagination in both science and fiction for decades, even before nanotechnology was scientifically realised: the concept of nanobots, removing diseased cells from our blood stream, is a very clear image in sci-fi literature, and excitingly it is actually something that we believe will become a reality in the coming years.

      This section is devoted to highlighting special properties of nanomaterials within the scope of this book, illustrating their functions and how these properties serve several emerging and well-known applications with selected examples. Given these special features, which may not be found in bulk materials, an entire chapter focuses on giving an introduction to various key characterisation techniques used for studying and measuring properties of nanomaterials. This is followed by a description of ‘top-down’ and ‘bottom-up’ current and emerging methods for nanomaterials synthesis and manufacturing along with the physicochemical principles of nanomaterial formation. Suitable examples of the chemistry of materials are provided to help illustrate the processes.

      IOP Publishing

      Green Nanomaterials

      From bioinspired synthesis to sustainable manufacturing of inorganic nanomaterials

       Siddharth V Patwardhan and Sarah S Staniland

      Chapter 2

      Nanomaterials: what are they and why do we want them?

      2.1 Fundamentals of the nanoscale

      Although the term ‘nanotechnology’ is commonly used beyond science by the general public and in the media, an understanding of what ‘nano’ is defined in length scale, and the changes to physical properties that occur in materials in this miniaturised world, are not generally realised.

      ‘Nano’ comes from the Greek word for ‘dwarf’ and is the prefix of a measurement that is ×10⁻⁹ (or one billionth of that unit). In the context of material science and nanomaterials we are interested in the measurement unit of length: metres, and therefore the nanometre (nm), although ‘nano’ can prefix any unit, for example nanosecond, nanogram or nanomole. Put into context, there are one million nanometres in a millimetre and a billion in a metre. Figure 2.1 may be used to aid visualisation of these different length scales. Considering it from the opposite side, scaling up rather than down, we measure atomic bond lengths in Angstroms (Å) which are ×10⁻¹⁰ m, and so the nanometre length scale is 10× larger than the length scale at which we consider atomic reaction (chemistry) to occur. For example, the unit cell (smallest crystal unit) of sodium chloride is 0.56 nm², containing four of each type of (sodium and chloride) atoms. Although performing chemical reactions at this scale is an old and well-established discipline, the idea of crafting materials and components on the nanoscale and designing and creating tailored materials for smart applications is now developing as the relatively new disciplines of nanoscience and nanotechnology.

      Figure 2.1. Schematic of length scale with respect to a nanometre, showing common examples. Source: Abhijit Tembhekar/LadyofHats/Alan Cann/Acharya KR, Fry E E, Logan D T, Stuart D I/; visualisation: Astrojan/Белых Владислав Дмитриевич.

      In 1959, Professor Richard Feynman delivered his seminal lecture entitled ‘There’s plenty of room at the bottom’, in which he imagined what could be achieved if we shrunk technology down to the nanoscale. This lecture inspired material scientists and technologists to challenge themselves to venture to the smallest length scales possible for materials. The lecture has thus been defined as the birth of nanoscience and nanotechnology. As a physicist, Professor Feynman was fully aware that this shrinking would not be a simple linear scaling issue, because smaller components will not display the same properties as the bulk material. Rather, materials at the nanoscale display rich and complex properties specifically dependent on their dimensions, proportion of surface atoms, and morphology (see the examples in the sections below).

      Nanomaterial can be defined as having at least one dimension in the nanoscale (between 1–100 nm). This ranges from thin nanometre films (one dimension at the nanoscale) to nanowires (two dimensions at the nanoscale) to nanoparticles (all three dimensions at the nanoscale). Dimensions smaller than 1 nm are in the territory of clusters, made up of a few atoms, which are outside the realm of nanomaterials. We should also consider that the nanoscale can apply to voids as well as matter, and thus nanoporous materials can also be considered as nanomaterials, particularly as they have large surface areas. While 100 nm is conventionally the upper limit, this can be blurred. Materials termed nanoscale are reported over this size, if the properties displayed are still within the realms of nanoproperties (discussed in section 2.2) or, (more frequently as time goes on) it suits to classify them as a nanomaterial for political (non-scientific) reasons!

      The nanoscale is a most interesting length scale for solid state materials for several reasons. First, from a philosophical viewpoint, it can be asked: how small can a particle be before it is no longer a material? Indeed, there is a valid scientific debate about whether or not a nanoparticle can or cannot dissolve, or if it is already a solute, when in a solution.

      Second, nanomaterials have a large percentage of surface compared to bulk material, and surfaces have very different chemical and physical properties to the bulk. To illustrate this, let us consider two spherical particles with radii of 1 μm and 10 nm. The ratio of the external surface area to volume (A:V) of each particle is inversely proportional to its radius (figure 2.2):

      AV=4πr243πr3=3r.(2.1)

      For these two particles, A:V will be respectively 0.003 nm⁻¹ and 0.3 nm⁻¹. This means that the 10 nm particle will have 100 times more surface for the same volume. As a result, the 10 nm particle will have 100 times more atoms on the surface available to interact with the external environment. This phenomenon leads to significantly increased catalytic activities, for example. This effect also manifests into other special effects and properties such as optical and magnetic properties, which are discussed further below.

      Figure 2.2. Graphical representation of how surface area increases with decreasing radius of a spherical particle (on a log 10 scale).

      Third, the nanoscale lies at the boundary of bulk material properties and atomic chemical properties (figure 2.3). The former is generally described by continuous transitions, while the latter must be considered as quantum mechanical. Quantum mechanics is both difficult and abstract to conceptualise but can be linked to the macroscale world by the theory of wave particle duality, where any wave can also be described as a particle with the same energy, and vice versa.

E=hf=ρc.(2.2)

      These are linked by the work of both Einstein and Broglie, that shows that a particle and wave with the same energy are interchangeable (equation (2.2), where E = energy, h = Planck’s constant, f = frequency of the wave, p = particle momentum = mass × velocity, (momentum of a particle) and c = the speed of light). At this stage however, it is simple to see that properties at the atomic scale are quantised because electrons (the principle particle (or wave) behind all chemistry) reside in discrete quantised electronic orbitals (defined by having an integer wavelength). This is simply described as if one considered the energy a particle can have in a defined narrow