Green Nanomaterials. Siddharth Patwardhan
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which is the energy that equates to any integer wavelength that will fit in the box as a standing wave (figure 2.3). These energy levels are equal distance apart as the box width remains constant (figure 2.3), however, the energy well of an atom follows a curve and as such the energy levels become closer together the higher the energy (figure 2.3). This is also directed by the shape of the energy well; the deeper the well, the further apart the energy levels, while the value of each energy level is also directed by the element (the pull of the nucleus etc). Thus, there is only one series of discrete energies that relates to that electron and its energy levels, of that particular atom. As the number of atoms in a material increases, so too does the number of interactions between atoms and electrons. The more interactions, the harder it is to define discrete energy levels as more and more energy levels occur, forming a continuum which is what we can describe in a bulk material (figure 2.3). One can similarly conceptualise the particle in a box on a macroscale. The width of the box is now much greater and the wavelength associated with a macroscale (large mass) particle is now so small, the particle can now be considered to occupy any energy, so it becomes a continuous system. In Newtonian physics a particle can have any energy, velocity and thus momentum within its box (figure 2.3). The interface where quantum states transition to continuous phases is at the nanoscale and this leads to a range of exotic behaviours, some of which are briefly described below in section 2.3.
Figure 2.3. Depiction of how nanoscale materials lie at the crossover between the atomic quantum laws of physics and the macroscale Newtonian laws of physics. In quantum physics: the left shows how a particle in a very narrow box can only have the energy quantised by the size of the wavelength. The change in energy of these is the same as the box sides are parallel. For an atomic system, the change in energy between levels decreases at increasing number of n, due to the energy potential. Conversely, a particle in a macroscale box can move in any direction at any given energy, so there is a continuum of energy a particle can possess.
2.2 Tangible and historical examples of nanomaterials
It is no coincidence that nanoscience has grown in parallel with the growth and development of instrumentation, imaging and spectroscopic techniques that have the resolution to allow probing on the nano- to the atomic scale (see characterisation in chapter 3). This has been essential to analyse and understand these materials as well as aid design and synthesis. However, while we may not have been able to visualise or understand them, we have been using the properties of nanomaterials for millennia, with examples stretching through history (figure 2.4). One of the earliest examples is the 4th century Roman Lycurgus cup. This beautiful glass cup, mounted in a carved bronze exterior, is now housed in the British Museum. It is an excellent example of nano-optics (specifically surface plasmon resonance), where, because it contains different metallic nanoparticles it looks green if illuminated from the front, because silver nanoparticles scatter the reflective light, and red if illuminated from behind, due to surface plasmon resonance from the gold nanoparticles interacting with transmitted light. It was not until 1847 that Faraday first reported nanoparticles; gold nanoparticles, as a colloidal suspension, he called a ‘gold sol’. He was the first to recognise and describe the exotic optical properties gold nanoparticles possess, which is perhaps the first recognition of quantum size effects and thus quantum mechanics. Not quite so complex but equally dramatic and awe-inspiring, stained glass windows have been used for centuries in religious buildings. Again, metallic nanoparticles are responsible for the incredible colour still enjoyed today in these and also in ceramic glazes, centuries after they were created. The photo sensitivity of silver was developed during the 1800s to invent photography and film (the origin of the term ‘silver screen’) as we knew it up until the late 20th century. While digital imaging has now overtaken film to become the norm, many professionals still prefer traditional film for its distinctive look, and also for its better resolution. Colloid silver on film has nanoscale resolution! The later 20th century saw the explosion of nanomaterials, both in their own right (the subject of this book) and for inclusion in many consumer products, such as solar photovoltaic materials in solar panels and tyres. We even now see silver nanoparticles embedded into socks to for their antimicrobial properties, to stop them smelling! In the present day, our understanding of nanoproperties and ability to make and characterise nanomaterials is growing, and so too is our ambition for their use. The field of nanomedicine has been explored in recent years, and is the subject of section 2.4.1. While curing disease is one of the key applications, it is also the aim of nanoscientists globally to create the smallest working nanomachines, using classic engineering shapes such as axials, rings and wheels at a molecular scale, to make nanowalkers, molecular motors and nanocars [1].
Figure 2.4. Schematic of nanoscale materials and accompanying timeline of their development.
2.3 Special properties offered by the nanoscale
There are many physical properties that are affected at the nanoscale, such as melting point and electronic properties. Furthermore, due to the large surface area and predominance of surface chemistry, nanomaterials are also ideal for heterogenous catalysis. The purpose of this chapter is NOT to give a complete, definitive account of all properties and applications of nanomaterials, but rather to give a flavour and thus must focus on a small selection of nanoproperties and their subsequent application.
There are two features of nanoscale materials: (1) increased surface-to-bulk volume ratio and (2) the reduction in size, which leads to the surface and quantum mechanical properties predominating over the bulk properties. Both key features are beautifully illustrated in turn by considering two optical properties of nanomaterials: the surface effect of surface plasmon resonance (SPR) and the quantum mechanical effect of quantum dot fluorescence. Furthermore, the reduction in size also affects magnetic properties. These three nanoproperties will be described in this section, with the following section showing real-world applications of these specific properties.
2.3.1 Optical: surface plasmon resonance
A surface plasmon is the interaction of the free surface electrons of a metal with electromagnetic radiation. Surface plasmons are mostly seen in inert noble metals, but all metals and even some highly doped semiconductors will exhibit a surface plasmon effect. When a metal surface is irradiated with light of a larger wavelength than the size of the particle, the conducting electrons will polarise and collectively oscillate about the surface of the particle in response to the wave, and are thus driven to form a resonance standing wave, giving a resonance peak (figure 2.5(i)). This is known as SPR. At this peak the electromagnetic radiation is both scattered and absorbed by the metal [2]. Some of the radiation is absorbed by the nanomaterial and converted to crystal lattice vibrational energy (phonons), while the rest of the radiation is emitted in different directions (scattering).
Figure 2.5. (i) Schematic of how electromagnetic wave interacts with surface electrons on a nanoparticle (adapted from figure in [2] with permission of the Royal Society of Chemistry). (ii) Regions of the electromagnetic spectrum that specific nanomaterials of different morphologies emit due to SPR (adapted from figures in [3] with permission of Springer).
The colour of the light emitted comes predominantly from the scattering, so we should consider what contributes to the SPR scattering. We need to think about polarisability; the ease with which charges (the conduction electrons) on the nanoparticle surface undergo charge (re)distribution and form partial dipoles. This is dependent on how far the electrons need to move, i.e. the cross-sectional area (so in turn, the radius of the particle (r)). It also depends on the nanoparticle material, and how easy electrons can move on this metal: the dielectric