**4.2 Indium oxide based bulk nanomaterials**

Functional properties of bulk nanomaterials can sometimes be advantageous compare to nanostructured thin films and the properties such as structural, optical, optoelectronic, microstructural and electrical in bulk nanomaterials differ from their respective thin film counterpart. In this respect, the preparation methods that govern the generation of specific structural features, porosity and defects in the bulk nanomaterials. It is reported that ITO and nickel doped ITO nanomaterials exhibit X-ray diffraction peaks similar to that of pure In2O3 with cubic bixbyite structure [27–30]. To calculate the band gap energy (BGE), the optical reflectance spectrum of a bulk nanomaterial can be recorded and then converted into its absorption spectrum. The absorption coefficient (α) can be determined using Kubelka-Munk function relation, α = (1 − R)<sup>2</sup> /2R. The BGE (Eg) can be determined from the relation, αhυ = A (Eg − h)1/2, where h, υ and A are Plank's constant, frequency of light, proportionality constant, respectively [57, 61]. The BGE value of a metal doped IO/IO based nanomaterial varies depending on the nature of dopant element [27–30]. On the other hand, magnetic properties of IO/IO based nanomaterials are of great interest for basic science. Pure In2O3 shows diamagnetic behavior while the ITO displays ferromagnetism at room temperature. Moreover, different transitional metal doped In2O3 nanomaterials exhibit ferromagnetism at room temperature due to presence of oxygen vacancies [59, 60].
