Page 44 - Synthesis of Functional Nanoparticles Using an Atmospheric Pressure Microplasma Process - LiangLiang Lin

Chapter 2
artifacts in the final image. However, it only scans a much small area than SEM, and cannot be applied for chemical identification and mapping.
2) Chemical composition
Chemical composition is the most important parameter that determines the properties of nanoparticles. For example, Au nanoparticles behave unique optical properties that cannot be achieved by Fe, Cu, Zn, and Al nanoparticles.27 The incorporation of Fe into Ni nanoparticles can enhance its catalytic activity for CNTs growth to a large extent.16 There are also reports using metallic alloys for the biomedical applications, where two or more types of nanoparticles are combined together for specific performance.28,29
Two most widely used techniques for characterizing the chemical composition of nanoparticles are energy dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS). The EDX measurement is conducted by focusing a high-energy beam of electrons on the sample, which may excite the electrons of the sample and eject them from the inner shells of atoms. Meanwhile, electron hole was generated where the electrons were. Then electrons from an outer, higher-energy shell automatically fill the hole and release energy in the form of X-rays. Since each element has a unique atomic structure, the energies of the X-rays are relevant to the energy levels of atoms and can be used to identify the elemental composition.30 Moreover, the energy densities are related to the element concentration, by scanning the beam over a specific area, this technique can also estimate the relative abundance of elements and their spatial distributions.31 By contrast, for XPS analysis, a beam of X-ray photons is irradiated to the sample and transfers the energy to core-level electrons. The electrons can be ejected from initial states, with kinetic energies dependent on the incident X-ray, the binding energy of the atomic orbits and the chemical environment of the originated atoms. A photoelectron spectrum is recorded by counting the ejected electrons over a range of electron kinetic energies. The analysis of the energies and intensities of the photoelectron peaks enable identification and quantification of all the present elements.
Compared to the EDX characterization, a distinctive advantage of XPS is the ability to get information of chemical state of atoms (e.g. the oxidation state of element), which has practical importance in many areas. However, due to the limited escape depth of electrons, XPS can only detect signals from sample depth of <10 nm. Therefore, it is considered as a surface characterization technique instead of bulk material analysis. Additionally, in order to achieve adequate chemical specificity and spatial resolution, XPS requires ultra-high vacuum (P < 10−9 millibar) during measurements. In this thesis, both EDX and XPS techniques are employed to get complementary information of the obtained nanoparticles.
In addition to the EDX and XPS, there are also various techniques for the characterization of chemical composition, such as X-ray fluorescence (XRF), inductively coupled plasma mass spectrometry (ICP-MS), inductively coupled plasma optical emission spectroscopy (ICP- OES) or atomic absorption spectroscopy (AAS). However, they are not so frequently used compared to the EDX and XPS for the characterization of nanoparticles, thus is beyond the scope of the present study.
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