Evolution of tin oxide (SnO 2 ) nanostructures synthesized by hydrothermal method

Tin oxide (SnO 2 ), a versatile metal oxide due to its wide range of applications and its nature as an amphoteric oxide, has attracted researchers globally for many decades. Hydrothermal synthesis of wide band gap oxides with controllable nano shape and size is of primary attraction leading to myriad areas of applications such as electrodes in Lithium-ion batteries, gas sensing, photo-catalyst etc. to name a few. In this work, we have synthesized different types of nanostructures of Tin oxide through low temperature(180 o C) Hydrothermal process by varying the concentration of its precursor solution (SnCl 4 .5H 2 O) from 0.0625M to 0.25M. The characterization of as -Synthesized SnO 2 done using UV-Vis spectroscopy, Scanning Electron Microscopy (SEM), Energy Dispersive X ray (EDX) and X-Ray Diffraction (XRD) conrm synthesis of tin oxide and formation of various nanostructures as a function of concentration of the precursor solution. The evolution of various shapes of nanostructures has been discussed in light of existing theories.

In recent past, attention has been paid to SnO 2 nano-structures, in particular their hierarchical morphology due to their interesting physical and chemical properties. Numerous studies on tin oxide shows that we can tune the experimental parameters like solvent, pH, concentration, temperature, surfactant and time [15][16][17][18] to in uence the morphology and structure of the nal product. The crystal growth through a solution route can be controlled by the preparation conditions such as the introduction of additives, the solvent, precursor concentration, reaction temperature etc. It is well known that many experimental parameters such as the solvent, the solution pH, the OHconcentration have a profound in uence on the morphology of SnO 2 architectures. Many earlier studies were successful in preparing different morphologies of SnO 2 and in inducing characteristics related to these morphologies like Wang et.al [4,19] study on the synthesis and gas sensing of SnO 2 nano-rods and hollow micro-spheres, Yang et.al [20] study on self-assembled 3D ower-shaped SnO 2 nanostructure. Supothina et al. [16] reported the synthesis of SnO 2 nanorod clusters via hydrothermal route using different concentrations of precursor, Na 2 SnO 3 .3H 2 O and NaOH at 200 o C for 48 h. Furthermore, a few studies report on the synthesis of SnO 2 nanostructures using surfactants. Cao et al. [21] produced SnO 2 nano cubes and nanospheres using sodium dodecyl sulphate (SDS) as a surfactant via hydrothermal route at a temperature of 180 o C. But the usage of surfactants can cause environmental risks, especially to aquatic species. In another study, Matin et al. [22] successfully prepared SnO 2 nanoparticles without the use of any alkaline solution and template; however, the samples required further calcination at an elevated temperature of 350 o C following the hydrothermal process at 130 o C. Most of these studies are concerned with the synthesis SnO 2 nano-material in discrete morphologies by different methods. In this work, we have synthesized SnO 2 nano -spheres, nano-rods and nano-owers by adjusting the precursor concentration via a simple hydrothermal route at a temperature of 180 o C using a mixture of water and ethanol as a solvent. This method does not need any surfactant, template or calcination to control the shape and the size of particles. In contrast, a concentrated sodium hydroxide solution was used to control the degree of hydrolysis during the synthesis of SnO 2 nanostructures. The effect of precursor concentration on morphology of SnO 2 is investigated and the evolution of various nanostructures of Tin oxide is studied and explained.

Results And Discussions
Optical characterization of the as-synthesized as well as hydrothermally processed samples has been done using UV-Vis spectroscopy. Morphology, size distribution and composition of the hydrothermally processed samples is determined using SEM and EDAX Analysis. Phase structure and phase purity of the as-synthesized nano-structures is determined using XRD. Some of the results obtained for these characterizations are discussed as under:

UV-Vis Measurements
The UV-Vis Spectrophotometry was done before and after hydrothermal synthesis process. Fig.(1a) and 1(b) shows the result obtained for the three different concentration of precursor solutions. As seen from g.1(a), there is an upsurge in absorption from 190nm and the total absorption region extends up to 375nm for all the three as prepared samples. This is in contrast to the absorption region seen for hydrothermally processed samples as seen in g.1(b). The absorption region is squeezed to a smaller region limited to around 275 nm with increase in absorptance peak value. This is indicative of the formation of different types of nanostructures as smaller particle size will absorb higher in shorter wavelength region.

XRD
The crystalline phase of hydrothermally synthesized SnO 2 samples were analyzed by XRD, and the typical XRD patterns for two compositions 0.0625M and 0.167M are shown in Fig. 2 Table 1. It can be seen that the weight percentage of tin substantially decreases when the precursor concentration increases from 0.0625M to 0.167M. It is followed by a substantial increase in tin weight percentage when precursor concentration is further increased to 0.25M.The corresponding reverse trends is observed in the weight percent of the oxygen.

SEM Analysis
The SEM images of the three samples with different concentrations of precursor solution is shown in g.4(a) to 4(d). Fig.4(a) shows the SEM morphology of hydrothermally processed SnO 2 sample with precursor concentration of 0. 0625M.The presence of numerous numbers of nanospheres can be easily seen. The formation of nanorods can be seen in g. 4(b). The length of the nanorods is of the order between 800-900 nm with an average diameter of around 400 nm. Fig.4(c) and 4(d) show the evolution of ower like nanostructure morphology. On close examination of g.4(d), it can be clearly seen that the ower like morphology as the result of well-arranged nanorods. As seen in the XRD results, the crystalline behavior is enhanced with increase in precursor concentration with a corresponding increase in the crystallite size.

The SnO 2 nanostructures formation mechanism
Based on the obtained results, we propose a possible formation mechanism for SnO 2 nano-particles: The ratio of Sn + //OHplays an important role in the evolution of various nanostructures like nanospheres, nanorods and nano owers as elucidated by EDAX results. Nucleation leads to the formation of nanospheres which is followed by crystal growth leading to formation of nanorods and then agglomeration in form of nano owers occurs with precursor concentration [23].

Conclusion
Synthesis of various nanostructures of SnO 2 like nanosphere, nanorods and nano owers have been experimentally performed by a simple hydrothermal route. The morphologies and shapes of the nanostructures found to be tuned by varying the concentrations of precursor solutions. A possible formation process and growth mechanism for such synthesized SnO 2 nanostructures have been studied.
The effect of precursor concentration on morphology of SnO 2 is investigated and the evolution of various nanostructures of Tin oxide is studied and explained.

Declarations
Competing interests: The authors declare no competing interests.