Techniques and Instruments Used to Study Nanomaterials
The current boom in nanoscience and nanotechnology is to some extent a result of the recent advances in the methods of investigating and manipulating small objects. Such developments were mostly driven by the improvement in the resolution, reliability and availability of electron microscopy, including scanning (SEM) and transmittance (TEM) modes. The recent invention of scanning tunnelling (STM) and atomic force (AFM) microscopy, first appearing in 1982 and 1986, respectively,1 also provided additional tools for probing and manipulating atomic scale objects.
SEM, AFM and STM instruments detect the surface features of nanos-tructured objects. However, when it comes to nanotubular morphology, sometimes the only method which can differentiate between the tubular or rod shape of objects is TEM imaging, which allows us to "see" the projection of the object. Furthermore, in order to avoid misinterpretation and artefacts associated with TEM imaging it is necessary to detect the nanotubes oriented parallel to the electron beam casting the projection in the shape of a circle. Although electron microscopy imaging allows us to directly detect the shape and dimensions of the objects with great accuracy, the disadvantage is that it covers only a limited area of the sample. As a result, there are always doubts as to how representative the imaged object is of the whole sample.
One of the methods which measures macroscopic parameters and associates them with microscopic parameters, is the method of gas adsorption into the porous samples. The most popular and developed method is nitrogen adsorption, which has been widely used in the investigation of catalysts for the characterisation of porous materials. In typical experiments, the adsorption of gaseous nitrogen on the surface of a material is studied at a temperature of -196 °C, in the range of relative pressures from 0 to 1. Using an adsorption model, it is possible to determine the specific surface area (BET) and the pore size distribution (BJH). These data can then be associated with the morphology and size of the nanostructure. The advantage of the method is simplicity, cost and reproducibility. The disadvantage is that it requires a model which can associate pore size distribution with the dimensions of the nanostructure. This approach is usually applicable only to nanostructures from the same family (e.g. nanotubes, nanoparticles etc.). Section 3.2 considers the porous structure of titanate nanotubes.
The degree of atomic order in nanostructures can be studied using electron or X-ray diffraction methods. There are some difficulties with the interpretation of diffraction data, due to the relatively small size (or polycrystallinity) of nanostructures resulting in a widening of the reflexes. Furthermore, some nanostructures (e.g. as-prepared TiO2 anodized nanotubes) may be amorphous. In Section 3.1, crystallographic studies of titanate and TiO2 nanotubes are reviewed.
The electronic structure and the nature of the surface atoms has been actively studied using conventional spectroscopy methods. Section 3.3 considers various spectroscopic data. Results from UV/VIS, PL, ESR, XPS, NMR, Raman and FTIR spectroscopy are interpreted.
The methods of computational chemistry are also widely used in nanoscience. In the instance of titanate nanotubes, however, their use is yet limited due to the massive number of atoms in its nanostructure, resulting in a huge basis set of atomic orbitals. For example, a typical 100 nm long titanate nanotube contains more than ten thousand titanium atoms, making direct ab initio calculations impossible using current computing facilities. However, future developments in computing technology and the implementation of various model approximations would allow further simulations and would stimulate progress in the discovery of new nanomaterials.
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