**1. Introduction**

It is no doubt that nanomaterials have attracted significant attention for both basic and applied sciences because these materials in nanodimension (1–100 nm) exhibit novel features including high surface area, excellent physical and chemical stability and lower material density compared to their bulk counterpart. In fact, these features of the nanomaterials help the researchers to design and fabricate novel functional nanomaterials/devices for practical use. Today, various forms of nanomaterials such as quantum dots, nanoparticles, nanoflakes, nanobelts, nanoribbons, nanosheets, nanofilms, nanotubes, nanofibers even nanocomposites have

widely been used to improve the materials properties including thermal, electrical, mechanical, optoelectronics, corrosion resistant, self-cleaning, and sensing [1–3].

Over the past decades, nanostructured metal oxide semiconductors (MOSs) have drawn tremendous attention to materials researchers due to their widespread applications in various fields [4–6]. Among various MOSs, indium oxide (In2O3) has been investigated widely owing to its wide band gap, high electrical conductivity, stability and excellent optoelectronic properties [2, 7]. In2O3 (IO) is a wide band gap n-type semiconductor with direct band gap energy of 3.6 eV at room temperature [7, 8]. It is found that the band gap energy of IO thin film primarily depends on various factors such as annealing temperature and atmosphere as well as the nature of the substrate on which the film is to be deposited. The growth temperature also influences upon the morphological, structural, electrical and optical properties of IO based thin films. It is to be further noted that the thin films with enhanced functional properties like high electrical conductivity and visible transparency can be achieved by controlling the annealing temperature and atmosphere during the fabrication process [9]. It is noteworthy that these materials are suitable for different applications such as photovoltaic devices, liquid crystal displays, transparent conductive electrode in electronic devices, solar cells and flat panel displays, photodetectors, gas sensors, heat reflecting windows etc. [2, 8, 10]. On the other hand, different nanostructured IO based bulk nanomaterials such as nanosheets, nanowires, nanoparticles, quantum dots, single crystals are found to have potential applications [8, 11–13].

In the last decade, IO/IO based nanomaterials has been studied extensively. Around 54 years ago, Groth *et al.* [14] demonstrated that the small amount of Sn or Ti doping into IO can significantly enhance the electrical conductivity and infrared reflectivity without losing optical transparency in visible region. Based on this experimental observation, the Sn-doped In2O3, popularly known as indium tin oxide (ITO) creates an active area of research and development in the field of electrochromic and infrared reflective windows, light emitting diodes, transparent contacts for solar cells and flat panel displays and cladding layers for InGaNbased lasers [2]. Recently, the rapid increase in production of various electronic/ optoelectronic devices with ITO results a sharp increase in price of indium. In order to minimize the cost without sacrificing the functional properties, indium oxide based thin films have been fabricated [15–17]. In this respect, the formation of heterostructure with band gap engineering of IO or IO based nanomaterials improve its functional properties for advanced applications especially in transparent electronic devices and sensors [18–20]. In this regard, Wang *et al.* [18] reported hierarchically structured ZnO decorated with IO nanoparticles synthesized by one-pot sol-gel process towards improvement in n-butanol sensing performance. Moreover, N-doped graphene quantum dots modified three-dimensional ordered macroporous IO based nanocomposites had been fabricated for NO2 gas sensing application [10]. On the other hand, IO based nanomaterials are largely used for microelectronics and optoelectronic applications [2, 7, 19] and also found in significantly improved stability of solar cells without negotiating the performance of IO/ZnO electron transporting bilayer, synthesized by solution-process as reported by Kirmani *et al.* [20].

There are various methods now available to synthesize different types of IO/ IO-based nanomaterials. The common techniques to deposit the thin films of IO based nanomaterials are sol-gel, spray pyrolysis, Ink-Jet printing, physical/chemical vapor deposition and atomic layer deposition [2, 4, 8, 21]. On the other hand, IO-based nanomaterials are generally synthesized by sol-gel, solvothermal/hydrothermal, co-precipitation, thermal evaporation and solid state reaction methods [22]. *Indium Oxide Based Nanomaterials: Fabrication Strategies, Properties, Applications, Challenges… DOI: http://dx.doi.org/10.5772/intechopen.94743*

This chapter mainly highlights the synthesis strategies of IO based bulk nanomaterials with variable morphologies starting from spherical nanoparticles to nano-rods, nano-wires, nano-needles, nanopencils, nanopushpins etc. In addition, thin film deposition and periodic 1D/2D surface texturing techniques of IO based nanostructured thin films *vis-à-vis* their functional properties and applications have been discussed. Thus, the chapter covers a state-of-the-art survey on the fabrication strategies and recent advancement in properties of IO based nanomaterials with their different areas of applications. Finally, the challenges and future prospect of IO based nanomaterials have been briefly discussed.
