Synthesis of SnO2 Nanopowders for Advanced Ceramics and Electronic Sensor Transducer Devices and Characterization and Band Gap

Reverse Microemulsion Precipitation” was firstly developed for synthesizing SnO2 nanopowders were intended to as advanced structural materials and hazardous gases, particulates (Pb, Cd, Hg) sensing nanofabricated devices: sensor, transducer, MOSFET, electrodes. Prepared controlled nanopowders were encapsulated with oil phases in spherical water pole at water to surfactant mole ratio w0=8 and w0=10. Characteristic absorption of semiconductor at 303.4 nm and no absorption in higher λ and absorption edge in the 321.6-371.6 nm and band gap energy (3.6eV) were observed by UV-Vis measurement confirmed 2SnO2.4H2O nanoparticles is semiconductor. Sn-O stretching band at 678.94 cm -1 and no other groups presence confirmed complete removal of adsorbed chemicals in the course of calcination at 600°C about 4.0 hours from FTIR spectrum. XRD investigation found out phase pure tetragonal SnO2 nanocrystalline structures and average crystalline size 0.2380 nm at w0=8. SEM images exhibited spherical morphology counting average particle size 153.242 nm and 131.604 nm and average diameter 8.02 nm at w0=8 and 10.01 nm at w0=10 respectively. Higher specific surface area was observed 107.731 m / g (count 637) more than 86.314 m/ g (count 341) of relatively larger diameter which is more pronounced compared to ordinary Reverse Microemulsion Method. Findings and standards established this synthesis method as suitable for obtaining the higher degree of surface area and finest crystallinity.


Introduction
The chemical synthesis and characterization of SnO 2 nanopowders have received revolutionary attention from many researchers in diversified areas in worldwide [1]. SnO 2 is used as representative nanopowders and n-type semiconductor with high band gap 3.6 eV [2]. Since its high band gap, low electrical and magnetic properties, nano SnO 2 powders are used in several electronic applications as insulators, semiconductors and magnets, thin film etc. Regarding for its unique optical properties, it can be considered in transparent glass ceramics, transparent windows, transparent electrodes for solar cells, photoconductive devices in LCD, gas charging display, lithium ion batteries [3]. Due to brittleless, stretchable, high mechanical and thermal stress, the basic raw materials of automotive engines, cutting tools, flexible superconducting wire and fiber-optic connector components, wear and scratch resistant parts and watch glasses and ceramic coatings, ceramic membranes so on can be prepared and nanoceramic powders constitute an important segment of the whole national, international nanostructured market [4][5][6]. Because of antimicrobial, antifungal, antiviral properties, it is found to be used as bio-medical purposes e.g. bone grafting, dental implants, in normal water filter and ultra-pure water filter [7]. Moreover all of these aspects, its more efficient application found to be in nanofabricated sensor of environmentally hazardous gases (CH 4 , CO, H 2 S, CO 2 , NO 2 ), pollutants and particulates (Pb, Cd, Hg) [8]. For the efficiency of above mentioned products, nanosized SnO 2 nanopowders preparation method with high surface area is concerned [9].
Surplus research journals on synthesis and characterization of SnO 2 nanopowders found in literature till to date [10][11]. However there are few research papers available on advanced ceramics with SnO 2 nanopowders and on environmentally hazardous gases, particulates (Pb, Cd, Hg) sensor, transducer etc. devices. Many reported methods have been derived and employed to produce nano-scale SnO 2 materials in literature are microemulsions, sol-gel, high-energy ball milling, ultrasound, microwave irradiations, spray pyrolysis, direct strike precipitation, hydrothermal, chemical vapor deposition, laser synthesis, and self-propagating high-temperature synthesis, carbothermal reduction, plasma processes, solidstate reaction-thermal oxidation, solvothermal, aerosol flame deposition, polymerizing-complexing sol-gel method, polyol, oxidation, one step solid state synthesis, molten salt synthesis etc. [1-2, 12-13, 8, 14-18]. Various shapes of SnO 2 nanopowders e.g. spherical, nanotubes, nanotriangles, nanorods, hollow shperes, wormholes etc. have been observed in literature [19]. The enhance crystallinity and high surface area are of great importance and determinant for above mentioned products, method of synthesis should be developed.
In this work, method "Reverse Microemulsion Precipitation" was developed for synthesizing powders of SnO 2 nanoparticles are aimed to get overview of semiconductor of advanced ceramics components, structural materials, coatings and deserved for hazardous gases, particulates (Pb, Cd, Hg) sensing nanofabricated electronic devices: sensor, MOSFET, transducer etc. and provide to suppliers of advanced ceramic and structural raw materials, companies involved in R & D and commercialization and chemical companies interested in diversification. The nanopowders were synthesized taking w/o microemulsion as a microreactor. The characterization performed by UV-Vis Spectrophotometer, FTIR, XRD, SEM and the findings were studied comprehensively and to get insight of nanosizesd SnO 2 nanopowders herein.

Synthesis
Reverse microemulsions given in Table 1 at water to surfactant mole ratio, w 0 =8 and w 0 =10 were considered to be prepared by composed of and homogeneously mixing up cetyltrimethylammoniumbromide (CTAB), cyclohexane, 1butanol and de-ionized water using Microprocess controlled bench-top Ultrasonic Cleaner (model no. powersonic 410). Equal amounts of microemulsions at w 0 =8 were placed in two beakers and prepared 0.1M SnCl 4 solutions were injected into one beaker by pipettes and Microlitre Syringes (Hamilton Bonadus AG) ) and NH 4 OH (25%) solution directly incorporated into another beaker using pipettes until pH=12. Followed by homogeneous mixing of solutions of two beakers and pouring precipitating agent NH 4 OH solution until complete formation of 2SnO 2 .4H 2 O nanoparticles (opaque appearance) and reached maximum solubility of 2SnO 2 .4H 2 O nanoparticles at pH=12 ( until dark blue). Microemulsion at w 0 =10, mentioned process was done.
Thus prepared solution was flocculating by adding acetone and kept overnight for settledown of white precipitates of 2SnO 2 .4H 2 O and focused on centrifuging of upper chemicals: NH 4 Cl (aq), CTAB, cyclohexane thoroughly by droppers or in a superspeed centrifuge at 8500rpm or 10000rpm if available. The achieved hydroxide precipitates were washing using acetone and dried 24h in oven at 100 0 C to eliminate excess CTAB. The resulting precipitates proceed for calcination in Furnace (Nabertherm, geprufte Sicherheit) at 600 0 C for about 4 h to exhaust adsorbed chemicals and entirely conversion to SnO 2 nanopowders. The whole process was repeated at three times to obtain constant weight of SnO 2 nanopowders. Microemulsion at w 0 =10, mentioned process was done. The method followed the reaction: This method of synthesis was established on the basis of citation of references of characterization by XRD, UV-Vis Spectrophotometer, FTIR and SEM results consistent with standard SnO 2 nanopowders and commercial sample SnO 2 nanopowders. Commercial SnO 2 nanoparticles (SkySpring Nanomaterials, Inc., 50-70 nm) were characterized using the same techniques in the reported literature [20].

Characterization: (ASTM Standard Reference: Terminology for E2589-11 and Practices for E2651-13)
Optical transparency in the visible region (higher wavelength) and characteristic absorption in the UV region (short wavelength) of 2SnO 2 .4H 2 O nanoparticles in solution were scanned by using UV-Visible spectrophotometer (Shimadzu, model no.1650) and microemulsion (blank) of same composition used as reference solution and spectra was taken in the range 300-800 nm. XRD (Bruker D8 Advance) was subjected to characterize phase purity, shape, nanocrytalline structure and average crystalline size calculated using Debye-Scherrer relation and the diffraction pattern was recorded at 2θ:20 0 -70 0 using a CuKα1 radiation λ = 1.54060Å having Ni filter. The analysis of morphology (size and shape) was run out by conducting Scanning Electron Microscopy, SEM (Hitachi, model no. 33400). SEM was kept on about 12 hours before to measure. The % transmittance of SnO 2 nanopowders after calcination was traced in the wavenumber range 550-4000 cm -1 by operating automated FT-IR (Shimadzu, model no. iRaffinity-1) with a resolution of 4 cm -1 .

Results and Discussion
The UV-visible spectrum of 2SnO 2 .4H 2 O nanoparticles in solution at water to surfactant mole ratio w 0 =8 is to have presented in Figure 1 and displayed an intense characteristics absorption in the 303.4 nm wavelength range and an absorption edge between 321.6 nm and 371.6 nm and thus being have higher energy in consequence of the relatively larger photon energy than band gap energy to excite and transfer an electron to conduction band from valance band. These findings agree well with literatures [21][22]. However, the band gap energy of SnO 2 nanopowders was calculated by UV-Vis measurement and using Tauc relation [11]: where, Eg is the band gap energy and α is the optical absorption coefficient for nanoparticles. Similarly, calculated values for others wavelength is given in Table 2.
A plot of (αhυ)2 versus hυ called Tauc Plot shows linear region, the extrapolation of the curve gives Eg = 3.6 eV from the intersect of the hυ axis inserted in Figure 2 and excellent agreement with the standard Eg of SnO 2 nanoparticles and literatures [3].
The XRD patterns of SnO 2 nanopowders at w 0 =8 and w 0 =10 are inserted in Figure 3 [20]. Patterns from (111), (220) are weak due to signal to noise and low intensities. Author was not found other phases and thus confirmed formation of phase pure SnO 2 nanopowders by XRD. Broad peaks were observed in the XRD pattern for small size nanopowders at w 0 =8 compared to large nanopowders at w 0 =10 as a result of the smaller crystallite size, along with sharp peaks which bring forth the finest crystalline nature of average crystalline size was calculated from Debye Scherrer equation [23]: Where, D is the average crystalline size, K is the shape factor equal to 0.9, λ is the X-ray wavelength equal to 1.54060 A 0 , θ is the Bragg diffraction angle of the peak in degree.
And β is the full width at half maximum (FWHM) for peak broadening in 2θ unit..
When measuring FWHM using OriginPro software, its value was in 2θ (e.g. if measuring value 26.6748 in degree, then 2θ = 26.6748 = β), but during determination of average crystalline size, its value must be converted to radians ( 1 rad = 180 ×7/22 0r 180/π). The SEM micrographs of SnO 2 nanopowders sizes at w 0 =8 and w 0 =10 are displayed in Figure 4. The addressed images trace uniformly spherical particle size distribution. Reverse Microemulsion Precipitation is the method to get nanopowders with size and shape controlled and uniformly distribution of desired sizes nanopowders but there was observed some relatively larger particle sizes with approximately same diameter because of agglomeration of aggregates during calcination at 600 0 C about 4h and therefore, one cannot measure the particles size which is quite similar in shape with commercial sample and the prepared nanoparticles by microemulsion approach of prior literature [20,[24][25]. The findings was observed spherical morphology with average particle size 153.242 nm and 131.604 nm and an average diameter 8.02 nm at w 0 =8 and 10.01 nm at w 0 =10 respectively by analysis SEM image using Imagej software. Histograms are put in Figure 5 and the higher specific surface area was to be observed 107.731 m 2 / g (count 637) of small nanopowders at w 0 =8 and that of 86.314 m 2 / g (count 341) for relatively larger at w 0 =10 which is obviously more than conventional Reverse Microemulsion Method [1] are well agreement with XRD result.

Conclusions
Size controllable nano dimension SnO 2 powders synthesis method development has been major concerned in the last decades. "Reverse Microemulsion Precipitation" method was developed, characterized and found to be nanosized synthesized powders after reported literature for advanced ceramics, structural materials and sensor materials and so on.

UV-Visible
Spectrophotometer results demonstrate 2SnO 2 .4H 2 O nanoparticles in solution in the UV range exhibit an intense characteristic peak at 303.4 nm and absorption edge between 321.6-371.6 nm and in the visible range optically transparent. XRD patterns give the phase pure tetragonal crystalline structure of SnO 2 nanopowders and average crystalline size 0.380 nm for microemulsion at w 0 =8. The SEM image found to be described the spherical shape and average particle size 153.242 nm (count 637) and 131.604 nm (count 341) and average diameter nm 8.02 nm and 10.01 nm at w 0 = 8 and at w 0 =10 respectively of the SnO 2 nanopowders. FTIR record confirms only existence stretching band of oxide Sn-O of Sn-O-Sn at 678.94 cm -1 of prepared SnO 2 nanopowders by "Reverse Microemulsion Precipitation.