**2. Nanostructured metal oxide semiconductors**

Now-a-days, nanostructured MOSs draw special attention owing to their promising applications in various areas such as electronic, optoelectronic, energy storage and conversion, adsorption, catalysis and sensing for their fascinating characteristics including high surface-to-volume ratio, surface permeability, light harvesting capability, electrochemical and photochemical properties [4–6, 23]. Nanostructured materials can be classified as zero-dimensional (0-D, nanoparticles, core-shell nanoparticles), one-dimensional (1-D e.g. rods/wires), twodimensional (2-D e.g. layered structures, composite nanowires), and equiaxed or three dimensional (3-D e.g. nanotubes/nanowires bundles). It is to be noted that hierarchical nanostructures can be formed by combining 0-D, 1-D, 2-D and 3-D nanostructures [24]. These nanoscale structures of MOSs are capable to exhibit an improvement in mechanical, optical, electronic, optoelectronic or magnetic properties [1, 24].

#### **2.1 Indium oxide based nanomaterials**

In the next sub-sections, a discussion has been made on IO based nanomaterials especially bulk nanomaterials and porous nanomaterials including nanostructured thin films.

### *2.1.1 Bulk nanomaterials*

Development of functional nanomaterials in bulk form can fulfill the purpose of achieving some special properties which can not be possible in the form of thin film/coating. This is because the properties such as structural, optical, optoelectronic, microstructural, electrical etc. of a material in bulk form can greatly differ from its thin film counterpart. Thus, the fabrication of bulk nanomaterials is also highly essential for their widespread applications. In this respect, the method of their syntheses can determine the specific structural features related to grain size, interface boundaries, porosity, structural defects and so on [25–27]. In this regard, for the synthesis of some indium oxide based bulk nanomaterials few well-established methods and the applications of the products are listed in **Table 1**.

Nanomaterials with porous architecture are very much important in the field of nanoscience and nanotechnology because of the ability of the materials to interact with atoms, ions and molecules not only at their surface but also throughout the bulk region. Moreover, the surface area which is mainly dependent on the particle size, shape and volume of the void space present in a porous nanomaterial is directly related to the functional property [4–6]. Thus, to obtain superior functional properties, the textural properties should be tuned accordingly.


#### **Table 1.**

*Synthesis and application of IO based bulk nanocomposite materials.*

#### *2.1.2 Nanostructured thin films*

Indium oxide based nanostructured thin films have great significance owing to their variable band gap energy (3.2–3.8 eV) with high visible transparency, substantial environmental and chemical stability as well as high electron mobility and metal-like electrical conductivity [34–45]. These nanostructured thin films have been fabricated (**Table 2**) with excellent optical and electrical properties towards various applications [34–45]. However, the thin films are mostly used as TCOs [34–45]. In this regard, ITO thin film is known to be one of the most extensively used TCO. In addition, after modification of thin film surface (**Figure 1**) by periodic texturing adopting soft lithography or breath figure process (BRF), the surface textured films can be used for light frequency modulation, photoelectrochemical application and photocatalysis [4].


**Table 2.**

*Different methods of deposition and applications of selected IO based nanostructured thin films.*

*Indium Oxide Based Nanomaterials: Fabrication Strategies, Properties, Applications, Challenges… DOI: http://dx.doi.org/10.5772/intechopen.94743*

#### **Figure 1.**

*FESEM images of IO based nanostructured thin films: (a) zinc indium oxide, (b-d) ITO, (e) indium gallium zinc oxide and (f) 1D surface patterned zinc indium oxide [4, 46–49]. (Copyright reserved to the American Chemical Society (2017), AIP Publishing (2015), Springer Nature (2018) and AIP Publishing (2014) for references [4, 46, 48, 49], respectively).*
