Spectroscopy methods have been around for a long time now and have been widely used throughout the chemical, biological and engineering fields for many years. Since nanotechnology has developed into its own distinct area, many spectroscopy methods can now be used to characterize nanomaterials. While some have come from general chemistry, others have made the natural transition from measuring inorganic complexes to now being used on nanomaterials. In this article, we look at some of the most used and effective spectroscopy methods for characterizing nanomaterials– but it should be noted that this list is not exhaustive.
Spectroscopy techniques are a vital part of the characterization of nanomaterials, as well as other materials and molecules. They are often used in conjunction with various microscopy techniques to provide an overall picture of the nanomaterial in question. Where microscopy techniques are used to image the nanomaterial and provide a pictorial representation, spectroscopy provides a lot of information about the chemical structure and properties of a nanomaterial.
X-ray Photoelectron Spectroscopy (XPS)
X-ray Photoelectron Spectroscopy (XPS) is a widely used technique for nanomaterials, catalytic planes, inorganic structures and surfaces in general. XPS is usually used to determine the elemental composition of a nanomaterial, to see if there are any contaminants at the surface of the nanomaterial and in what quantities they exist, to determine the chemical states of a nanomaterial, to deduce the empirical formula of a nanomaterial, to determine the binding energies within the nanomaterial, to identify the density of electronic states in a nanomaterial, and to identify the depth of the surface layer in layered nanomaterials.
XPS uses monochromatic X-rays to penetrate a nanomaterial and this removes an electron from the valence band, or from within the p and d orbitals of an atom within the nanomaterial. The electrons are then ejected from the nanomaterial and pass through an ultra-high vacuum (UHV) chamber until they hit an electron energy analyser. The electrons are then characterized by their unique energy values and enables a complete picture of the nanomaterial to be built up.
Ultraviolet Photoelectron Spectroscopy (UPS)
Ultraviolet photoelectron spectroscopy (UPS) is often used in conjunction with XPS and for the same types of materials and applications as XPS. However, it does give different information to XPS despite being a relatively similar technique (and is why they are usually used together. UPS is often used to determine the electronic structure of a solid nanomaterial, how effective the nanomaterial is for adsorbing atoms to its surface, the degree of hybridization in the valence band, the position of the valence band (specifically the top of the band as UPS only gives the value of the Fermi level), as well as the nanomaterial’s electron affinity, ionization energy, activation energy and work function.
In UPS, the setup is very similar to XPS with the main difference being that ultraviolet (UV) photons are used to excite the material and eject an electron. However, the photons only penetrate at ¼ the depth of X-rays, so they will only eject an electron from the valence band. The electrons then pass along a UHV chamber and are analyzed using an electron energy analyzer. Again, the electrons are characterized by their kinetic energy values and each peak on the spectrum gives information about the type of bonding orbitals within the nanomaterial.
Raman spectroscopy is another well-used technique for the characterization of nanomaterials, although Raman spectroscopy is used throughout various fields. Raman spectroscopy relies on the principles of inelastic scattering (Raman scattering), i.e. when a light photon is scattered at a different frequency to the photon that hits the nanomaterial. A monochromatic wavelength of light is focused onto a nanomaterial and a filter collects all the wavelengths of light that have been elastically scattered (i.e. same wavelength as the incident wavelength). The remaining light wavelengths are collected by a charge-couple device (CCD) detector and analyzed.
Most characterization approaches involve the analysis of vibrational modes within nanomaterials and is particularly useful approach for oxygen-rich nanomaterials. It is also a technique that has found a lot of use with graphene and other carbon-based nanomaterials. Overall, Raman spectroscopy can be used to identify unknown materials by comparing the spectra against known examples, for identifying polymorphs, for tracking changes in the molecular structure and crystallinity of a nanomaterial, for determining the residual stress on a nanomaterial, for identifying the orientation of a nanomaterial, and for determining the intra and inter bond vibrations within a nanomaterial.
Ultraviolet-Visible spectroscopy (UV-Vis)
Ultraviolet-visible spectroscopy (UV-Vis) is a technique that causes a molecule to absorb a specific wavelength of light, whereupon an electron is excited to a higher energy level. Two types of wavelengths are used, a reference beam that doesn’t interact with the nanomaterial and one that has been specifically chosen to excite the material. The detector then records the ratio between these two light beams and determines the concentration of the nanomaterial. This is possible because, when the nanomaterial (or molecule, nanoparticle etc) is absorbing the light, the intensity of the transmission is lowered– which can then be used in a quantitative manner.
Given that the nanomaterial must be dissolved in a solvent for the analysis to take place, UV-Vis has limited applications in nanotechnology compared to other techniques but is quite often used for nanoparticles in suspension. In addition to determining the concentration of a nanoparticle suspension, UV-Vis can also be used to determine the color absorption properties of metallic nanoparticles (through plasmonic absorbance), as well as the sorption, diffusion and release properties of nanoparticles/nanomaterials.
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