Synthesis Characterization and Electrochemical Performance of Chromium Doped Tin Oxide

Chromium doped Tin oxide nanoparticles with chromium concentrations ranging from 1 to 5 wt% were synthesized by microwave irradiation technique. Standard characterization techniques were used to understand the characteristics of the nanoparticles obtained. X-Ray Diffraction (XRD) pattern depicted the tetragonal crystal structure for Cr doped SnO2 nanoparticles. From the results of crystallite sizes for various doping concentrations, it was observed that doping inhibits the growth of crystalline grains of SnO2. Scanning Electron Microscope (SEM) images confirmed the surface morphology modifications due to varying doping concentration of Cr, nanocrystallite showed extra agglomerated status with mesoporous structures. Energy dispersive spectrometer (EDAX) observations confirmed the doping of chromium ions in SnO2 lattice.  Other standard characterization techniques such as FESEM, TEM, HRTEM, FTIR, UV-Vis spectroscopic analysis were also carried out for the samples prepared. The electrochemical behavior of the sample was determined using Cyclic Voltammetry (CV) by scanning the potential at a rate of 50 mV s‾¹ and for a maximum current of 600 mA carried out on undoped SnO2 and Cr doped SnO2. It was observed that as the wt% of Cr in Cr doped SnO2 increases, the electrochemical performance increases as compared to undoped SnO2. A fairly larger peak current of 15 μA and a larger oxidation peak potential of 0.76 V were observed for 5 wt% Cr doped SnO2.

Chromium doped Tin oxide nanoparticles with chromium concentrations ranging from 1 to 5 wt% were synthesized by microwave irradiation technique. Standard characterization techniques were used to understand the characteristics of the nanoparticles obtained. X-Ray Diffraction (XRD) pattern depicted the tetragonal crystal structure for Cr doped SnO 2 nanoparticles. From the results of crystallite sizes for various doping concentrations, it was observed that doping inhibits the growth of crystalline grains of SnO 2 . Scanning Electron Microscope (SEM) images confirmed the surface morphology modifications due to varying doping concentration of Cr, nanocrystallite showed extra agglomerated status with mesoporous structures. Energy dispersive spectrometer (EDAX) observations confirmed the doping of chromium ions in SnO2 lattice. Other standard characterization techniques such as FESEM, TEM, HRTEM, FTIR, UV-Vis spectroscopic analysis were also carried out for the samples prepared. The electrochemical behavior of the sample was determined using Cyclic Voltammetry (CV) by scanning the potential at a rate of 50 mV s‾¹ and for a maximum current of 600 mA carried out on undoped SnO 2 and Cr doped SnO 2 . It was observed that as the wt% of Cr in Cr doped SnO 2 increases, the electrochemical performance increases as compared to undoped SnO 2 . A fairly larger peak current of 15 μA and a larger oxidation peak potential of 0.76 V were observed for 5 wt% Cr doped SnO 2 .

Introduction
Nanostructured metal-oxide semiconductor based sensors have wide applications in biological, environmental and analytical chemistry (1)(2)(3)(4)(5). Among the oxide semiconductors, tin oxide (SnO2) is one of the promising candidates as a host material that has been used in gas sensors, dye-sensitized solar cells, electrochromic windows, transparent conducting electrodes, transistors, catalysts and supercapacitors (6)(7)(8)(9)(10). SnO2 is a versatile material with a wide band gap (3.6 eV at 300 K) in its stoichiometric form, but due to lattice imperfections and oxygen vacancies arising during its production,it becomes n-type and conductive (9,(11)(12)(13). SnO2 material research has been of considerable interest because of its combined properties of plentiful oxygen vacancies, high optical transparency, chemical and electrochemical stability, good electrocatalytic activity, nontoxicity, good biocompatibility, and high electron communication features when doped (14). Chemical doping with appropriate elements (Fe, Cr, Co, Mn, Ni, etc.) is widely used as an effective method to tune surface states, energy levels of semiconductors and transport performance of carriers which enhances the electrical, electrochemical and magnetic properties of materials (12,14). The ionic radius of Cr 3+ is close to that of Sn 4+ (15,16), which means that Cr 3+ can easily penetrate into the SnO2 crystal lattice or substitute the Sn 4+ position in the crystal. Various methods have been used to synthesize the SnO2 nanostructures; Autoclave method, co-precipitation, pulsed laser deposition, spray pyrolysis, solid state reaction method, polymeric precursor's route, hydrothermal method etc. (17)(18)(19)(20)(21). However, it still remains a great challenge to develop a simple method for fabricating nano-SnO2, particularly metal ion doped SnO2 nanostructures with controlled morphology. Herein we report the synthesis of Cr doped SnO2 nanoparticles by a simple microwave irradiation method that takes only a few minutes to complete the reaction with prevented agglomeration. Results and discussions XRD Analysis  shrinkage of the unit cell volume is consistent with the fact that the ionic radius and valence of Cr³ + (63 °A) is smaller than that of Sn 4+ (74 °A). The XRD results showed that the Cr³ + ions incorporate into SnO2 lattice or replace Sn 4+ sites without changing the rutile structure. The average crystallite size (D) was determined using the diffraction peaks of (110) and (101) from Scherer's formula (1) (22,23). Where K is the shape factor whose value is taken as 0.89, λ is the wavelength of Cu Ka radiation, and β is the corrected full width at half maximum (FWHM) of the diffraction peak and θ is the diffracting angle. The average crystallite sizes of the synthesized nanoparticles were 17.88 nm (undoped), 26.41 nm (1% by wt), 28.50 nm (3% by wt) and 49.46 nm (5% by wt) respectively. These results indicate that the crystallite size of the Cr doped SnO2 nanoparticles increase as the doping concentration increases.
Scanning electron microscopy (SEM) analysis Figure 2 shows the SEM images of undoped and Chromium doped SnO2. It is observed that the prepared SnO2 particles are nanorods with some agglomeration, which may be due to annealing of SnO2 nanoparticles (NPs). However there is some non-uniformity in the shape and the existence of porosity. The measured mean particle size of the tin dioxide particles from the SEM image is 47.8 nm, which is comparable to XRD values determined for the particle size. A relatively uniform mixture of tetragonal like structures could be observed and the nanocrystallite showed extra agglomerated status with mesoporous structures.
Elemental analysis of NPs was done by using energy dispersive spectrometer (EDS) the plot of spectrum is shown in Figure 4. Emission peaks such as O and Sn observed in the EDS spectrum shows the presence of tin and oxygen elements and confirmed the stoichiometry of NPs.

Figure. 2: SEM images for undoped and Cr-doped Tin Oxide
Field emission scanning electron microscopy (FESEM) analysis Field Emission Scanning Electron Microscope (FESEM) was used to observe the morphology and structure changes. Figure 3 show that doping significantly alters the morphology of the nanorods. The surface of the SnO2 particles with 1% Cr doping are nano rods as shown in Figure 3(a), the grain size formed and surface modifications by increasing the concentration to 3 wt% is shown in Figure  3(b), modification of surface are also evident on increasing the Cr dopant level to 5 wt%, it is observed that the grain size formed on the surface of SnO2 film decreases on increase of Cr dopant (Figure 3(c)). Further large regular rectangular and triangular shaped grains are formed on the surface of SnO2 and the size of grains decrease with increasing dopant concentration. Energy dispersive analysis of X-Ray (EDAX) Figure 4 shows the compositional analyses of the Cr doped samples as depicted in the EDX spectra. The spectra show chromium peaks implying incorporation of Cr ions into SnO2 lattice. Furthermore, Hume-Rothery rules of substitutional solid solution suggest that substitutional incorporation of dopant is possible if the ionic radii of the host atom and the dopant must not differ by more than 5% (24)(25)(26). In this investigation, the ionic radii of Sn 4+ and Cr 3+ are 0.069 and 0.063 nm, respectively, which are well within 5% difference. This shows that Cr ions substitutionally replace Sn ions in the SnO2 lattice. Fourier transform infrared (FTIR) spectroscopy FTIR analysis is carried out in the wavenumber range from 450 cm -1 to 4000 cm -1 . The samples are with KBr, thoroughly mixed and pelletized by pressing under sufficient pressure, before FTIR analysis. FTIR spectra of SnO2 nanoparticles prepared at 600 °C are shown in Figure 5. The broad peak centered at 619 cm -1 is observed. The broad band between 800 and 500 cm -  Transmission electron microscope (TEM) analysis Typical TEM and high-resolution transmission electron microscope (HRTEM) images of undoped and 5 wt% Cr doped SnO2 samples are shown in Figure 6 (a & c) a spherical morphology with average size of 21nm and 16 nm respectively is observed the results are in good agreement with the estimated average crystallites size from the XRD pattern. The HRTEM image for undoped and 5 wt% Cr doped SnO2 nanoparticles are shown in Figure 6 (b & d). Undoped SnO2 nanocrystallites exhibited highly symmetrical and sharp lattice lines which confirm their single crystalline and defect free nature. However, Cr-doping seems to introduce twin-like defects in the crystallites as shown in Figure 6(d). Further, presence of any secondary phase or trace elements in the samples is not identified using HRTEM. Optical band-gap using UV-Visible spectrophotometer From the transmittance spectra shown in Figure 7 the Optical band gap values were obtained using the Tauc's formula (27). For undoped Sno2 band gap is 3.58 eV which is in good agreement with the reported values of Arun kumar sinha (11) it is observed that Cr doping has a positive influence on the temperature dependent of resistance. The electrochemical behaviour of the sample is determined using Cyclic Voltammetry (CV) by scanning the potential at a rate of 50 mV s‾¹ and for a maximum current of 600mA carried out on undoped SnO2 and Cr doped SnO2. From figure 8 it is observed that for undoped SnO2 (Figure 8(a)) the peak current is 11 μA for doping concentration of 1wt % the peak current is 12 μA for doping concentration of 3 wt% the peak current is 13 μA, as the wt% of Cr in Cr doped SnO2 increases, the electrochemical performance increases as compared to undoped SnO2, a fairly larger peak current of 15 μA and a larger oxidation peak potential of 0.76 V were observed for 5 wt% Cr doped SnO2.

Conclusion
Undoped and Cr doped SnO2 nanoparticles were synthesised using microwave irradiation method. The influence of Cr doping concentration on the structure, morphology, optical and electrochemical properties are reported. XRD patterns of undoped sample revealed the pure tetragonal rutile structure of SnO2. UV Spectroscopy shows decreasing band gap of SnO2 by addition of Chromium. The cyclic voltammetric (CV) studied confirmed that Cr doped sample have good electrochemical behaviour when compare to pure SnO2 sample.