Rapid Synthesis of SnO 2 Thin Films using Monoethanolamine through Wet Chemical Route

Objectives: To develop a unique wet chemical process for the rapid synthesis of SnO 2 thin ﬁlms and hence to study the structural, morphological, optical and electrical properties of the ﬁlms. Methods/Analysis: Polycrystalline SnO 2 thin ﬁlms having a thickness in the range of 800-1000 nm with crystallite size less than 5 nm were synthesized within a time of 20 minutes and without the need of post-annealing using SILAR technique. Lattice parameters, c/a ratio, cell volume, dislocation density, refractive index, extinction coeﬃcient and porosity of the SnO 2 thin ﬁlms were determined. Findings: Films have a coarse and porous surface morphology with very ﬁne pores distributed nearly uniformly on the ﬁlm surface. The crystallite size and strain developed in the SnO 2 ﬁlms were computed by the Williamson-Hall technique. Microstrain developed in the ﬁlms is of the order of 10 -3 . SnO 2 ﬁlms exhibit nearly 70% transmittance in the visible region. The optical band gap of the SnO 2 ﬁlms is 3.65 eV. The refractive index of the ﬁlms varies from 2.05 to 2.30 in the 450-1900 nm wavelength range. SnO 2 ﬁlms exhibit a resistivity in the order of 10 -1 Ω cm . Novelty : Reports a unique rapid wet chemical process for the direct preparation of crystalline SnO 2 thin ﬁlms using monoethaolamine at a temperature of 80 0 C. Literature survey reveals that no other method has yielded such type of good quality SnO 2 thin ﬁlms at this temperature and without post annealing or sintering. First time crystalline SnO 2 ﬁlms were prepared by a wet chemical process within 20 minutes.


Introduction
Wide band gap (3.6 eV) semiconductor tin oxide (SnO 2 ) is a promising material for various technological applications due to its high optical transparency in the visible range of the electromagnetic spectrum with excellent electric conductivity and high chemical stability (1)(2)(3)(4)(5)(6)(7) . SnO 2 exhibits high conductivity due to intrinsic defects like oxygen deficiencies whereas in the stoichiometric form it behaves like a good insulator. The electrical resistivity of undoped SnO 2 is limited to 10 −2 -10 −3 Ωcm and the conductivity of the films can be enhanced to a great extent by dopants (5) . SnO 2 films exhibit good field emission properties (1) . SnO 2 can be used as hole blocking layer for perovskite solar cells (6) . SnO 2 has been frequently used as resistive-type CO and H 2 gas sensors. The oxygen species with trapped negative charges existing in SnO 2 usually prompts an upward band bending and are hence promising sensing materials (2)(3)(4)7) . In TCO applications, both the surface and interface properties of SnO 2 plays a vital role (3,4) . The films have also been employed in a wide range of applications like oxidation catalyst, liquid crystal displays, light emitting diodes, thin film resistors, dye-sensitized solar cells, n-type ohmic electrodes for ink-jet printable electronics, photo-detectors (2)(3)(4)(5)(6)(7) .
SnO 2 thin films can be prepared by a variety of depositing processes, such as chemical vapour deposition (5) , sputtering (6) , spin coating (7) , thermal evaporation (8) , Pulsed laser deposition (9) , thermal oxidation (10) , spray pyrolysis (11) , sol-gel method (12) and Dr. Blade method (13) . These methods require sophisticated complex setups and are time consuming. Compared to these deposition methods, the successive ionic layer adsorption and reaction (SILAR) technique is a facile, environment-friendly and low-cost method, as well as appropriate for large area deposition of thin films with closely 100% material exploitation (14,15) . Moreover, SILAR opens up the possibility to control the particle size within the nanometer range and to vary the film morphology by varying the different parameters associated with it. On the other hand, there is little given in the literature about the preparation of SnO 2 films by a wet chemical process SILAR. Doping is relatively easy in this technique. Fabrication of good quality SnO 2 thin films at low processing temperatures is certainly a challenge.
In this work, we report a unique process for the rapid synthesis of adherent, dense and transparent SnO 2 thin films by SILAR on glass substrates using the cost-effective precursor tin (II) chloride (SnCl 2 .2H 2 O) and the complexing agent monoethanolamine (MEA). MEA (H 2 NCH 2 CH 2 OH) used in this work is a clear, water-white, hygroscopic liquid with a mild ammoniacal odour and having a pH of 11.8. MEA undergoes fair biodegradation and is not expected to be persistent in the environment. Hence, MEA can be treated as an environment-friendly complexing agent.
Fabrication of polycrystalline SnO 2 thin films within a time of 20 minutes at a low processing temperature of 353 K was achieved without the need for post-annealing or sintering. Films having a thickness in the range of 800-1000 nm with crystallite size less than 5 nm were deposited. Structural, morphological, optical and electrical properties were investigated to illustrate the quality of the films.

Materials and Methods
Analytical grade chemicals (Merck) were used as received. The details of successive ionic layer adsorption and reaction (SILAR) have been described elsewhere (14,15) . In this work, immersion time in cationic and anionic solution was optimized to 4 s. The optimized immersion time for rinsing the substrates was 8 s.
SnO 2 thin films were synthesized from the cationic precursor prepared by dissolving 1mg of SnCl 2 .2H 2 O in distilled water, followed by the addition of 3ml hydrochloric acid (35%) and 2 ml MEA. The cationic precursor solution was kept at room temperature. 2ml hydrogen peroxide (30%) mixed distilled water kept at 353 K served as an anionic precursor. Magnetic stirring was performed for thorough mixing while each of the chemical reagents was added to prepare the precursor solutions.
Films were synthesized by choosing 80 deposition cycles to get highly adherent films. Accordingly, the deposition time was almost around 20 minute. After deposition, the samples were rinsed in distilled water and dried under hot airflow before characterization.
The crystalline properties of the as-prepared and annealed SnO 2 films were examined by X-ray diffractometer Bruker AXS-8 advance and the surface morphology by JEOL Model JSM-6490LV microscope. The energy-dispersive X-ray (EDAX) spectroscopic measurements were done by the JEOL Model JED-2300 spectrometer, which was coupled with the scanning electron microscope. Optical spectra were recorded using a Hitachi-U-3410 UV-Vis-NIR spectrophotometer. The thickness of the films was determined by a gravimetric method using an analytical balance (OHAUS Model PA64C) of readability 0.1 mg. Electrical studies were carried out by the two probe method with electrodes made from silver paste using the Keithley source measure unit (Model SMU Keithley 2400).

Effect of MEA and film formation mechanism
SnCl 2 .2H 2 O used was dissolved in distilled water and followed by the addition of HCl acid solution and thereby adequate amounts of Sn 2+ ions were made available in the solution. Like triethanolamine, when MEA was added, it complexes with Sn 2+ to form a chelate compound [Sn(MEA) k ] 2+ (16) . MEA wraps around the Sn 2+ ions and averts the ions from further chemical changes or early release into the solution.
During the first SILAR process, on immersing the substrate in the cationic solution the [Sn(MEA) k ] 2+ complex was adsorbed on it. On the second process, i.e. when rinsing the substrate in distilled water kept at room temperature, [Sn(MEA) k ] 2+ on the substrate reacts with H 2 O and forms tin hydroxide (Sn(OH) 2 ). Excess and loosely held species on the substrate were removed during this process. In the third process, the strong oxidizer H 2 O 2 present in the distilled water kept at 353 K was oxidizing (Sn(OH) 2 ) into SnO 2 and the excess H 2 O 2 was simultaneously removed at this temperature on exposure to air. In the last immersion process, loosely held chemical species were removed from the film surface. Rapid deposition of SnO 2 films was achieved within 20 minute, which is a noticeable property of the unique MEA bath used here. This was well supported by a report of the rapid deposition of ZnO films from a chemical bath containing MEA (16) . SnO 2 film formation reaction may be as follows:

Structural and morphological analysis
The X-ray diffraction (XRD) profile of the SnO 2 thin films is depicted in Figure 1. These films were not treated after deposition. Literature study reveals that no other wet chemical method had yielded such type of crystalline films at a low processing temperature of 353 K without extensive post-annealing or sintering (1,7) . Moreover, crystalline quality of the films are in par with the films prepared by other sophisticated methods at high processing temperature (2,3,6) . For estimation of the crystallite size 'D' of the films, the Scherrer formula was used: Where 'k' is the shape factor which is 0.94, 'λ ' is the wavelength of X-rays which is 1.5406 nm for CuK a , 'β ' is the full width at half maximum of diffraction peak measured in radians and 'θ ' is the Bragg's angle. The grain size estimated from the main peak (110) is 4.7 nm and the value is in tune with the reported values obtained from other wet chemical methods (4,11) . The reported crystallite size varies between 4.796 nm to 11.563 nm in the case of films prepared by Spray Pyrolysis (2) . When the grain size of the films drops under 10 nm, accordingly, the size effect in electronic, optical and sensing properties can be anticipated (17) .
The primitive cell volume (V c ) was calculated using the equation, The obtained value of lattice parameters a = 0.4683 nm and c = 0.3124 nm matches with the ideal values a = 0.4738 nm and c = 0.3186 nm. The primitive cell volume computed is 0.0685 nm 3 and is slightly less than that of the ideal value 0.0715 nm 3 (17) . The crystallite size and strain developed in the SnO 2 films were further determined by the technique developed by the Williamson and Hall (W-H plot) (18) . The W-H equation is given as Eq. (7).
Where 'ε' is the strain developed in the films. W-H plot for the films was constructed by plotting ' ′ β cosθ ′ vs ′ 4 sinθ ′ and is shown in Figure 2. The crystallite size and strain were determined from the intercept at the 'β cosq'-axis and the slope of the W-H plot respectively according to Eq. (7). The microstrain extracted from the W-H analysis is the non-uniform strain resulting from the systematic shifts of atoms from their ideal positions. This shift may be originating from defects like point defects, site disorder, vacancies and also due to plastic deformation (19) . Microstrain associated with the films was further determined using Eq. (8) directly. The microstrain obtained from the W-H plot and Eq. (8) is plotted for comparison and is illustrated in the inset of Figure 2. The reported value of microstrain is between 6×10 -3 and 12×10 -3 (20) . The value of strain obtained is small and compressive (negative slope of W-H plot) in nature. The grain size obtained from the W-H plot is slightly less than that determined using Scherrer's formula, but establishes lower crystallite size and hence the possibilities of the size effect (17) . Figure 3 shows the grain size obtained from both techniques. https://www.indjst.org/ The defects in the SnO 2 films were quantified by computing the dislocation density (δ ) using Williamson Smallman relation (21) , The dislocation density of the films was determined using the grain size obtained from both Scherrer's formula and the W-H plot.  (2,20) . The values obtained in the rapid synthesis are also in par with this. The smaller value of dislocation densities may be an indication of the crystalline nature of the SnO 2 films. Figure 4 illustrates the scanning electron microscopic (SEM) images of the SnO 2 films. Some aggregated structures with undefined morphologies were observed. However, the films have a coarse and porous surface morphology with very fine pores distributed nearly uniformly on the film surface. This kind of dense morphology is capable of affording a large inherent adsorptive surface area. SEM images divulge agglomerated grains, which contributes to an augmented surface-to-volume ratio, making the films potential ones in the field of gas sensing (17) . The compositional purity of the SnO 2 films has been studied by EDAX analyses (not shown here), which indicate that the films contain mainly Sn and O, with no significant signal from other elements being detected. https://www.indjst.org/

Thickness of the films
Film thickness was determined by the gravimetric method. The samples were weighed before and after the film deposition to determine the mass of the films. Knowing the dimensions of the substrates used, the film thickness (t) was computed using Eq. (10).
Where 'm' is the mass of the film deposited on area ' A' and 'p' is the bulk density of the material. The bulk density of the material is assumed as 6.99x10 3 kg/m 3 . The thickness obtained for the films was 900 nm.

Optical properties
The optical transmittance and reflectance spectra of the samples are shown in Figure 5(a). The SnO 2 films exhibit nearly 70% transmittance in the visible region. The transmittance of the films is decreasing with an increase in wavelength in the nearinfrared (NIR) region of the spectrum. Optical transmission through a rough surface is much affected by the scattering of light having wavelengths near the magnitude of the film surface features (22)(23)(24) . The reflectance spectra of the SnO 2 films in Figure 5(a) shows reflectance below 20% and exhibits a slight increase in reflectance with an increase in wavelength in the NIR region. According to inter-band absorption theory, the relation between the absorption coefficient near the threshold and the incident photon energy for allowed direct transitions is given by the following relation (25) : Where ' A' is the probability parameter for the transition and 'E g 'is the optical band-gap energy. In Figure 5(b), (αhν) 2 versus hν plot of the films is shown. The value of 'E g ' was determined by extrapolating the linear region of (αhν) 2 vs hν plots. The 'E g ' value of the SnO 2 films obtained is 3.65 eV. The 'E g ' values obtained were in good agreement with the reports that SnO 2 films usually exhibit band gap energy in the range of 3.4 -4.6 eV (2,(26)(27)(28) . Optical band gap of the films can be tuned by suitable doping or annealing (2,11,29) Refractive index of the material influences the film optical characteristics such as transmission spectrum, reflection spectrum etc. The Refractive index, n, of the films was determined from the extinction coefficient 'k' and the reflectance 'R' using Eqns. (12) and (13) and is found in agreement with values reported in the literature (29)(30)(31)(32) .  Figure 6 shows the variation of the refractive index of the SnO 2 films as a function of wavelength. The values of 'n' obtained for the films vary from 2.05 to 2.30 in the 450-1900 nm wavelength range. A small gradual increase in 'n' can be noted with an increase in wavelength in the visible-NIR region and attains a steady value in the NIR region of the spectrum. The higher transmission in the visible region shown by the films may be due to the low refractive index possessed by it. The values of the refractive index obtained are in agreement with the reported values (26,31,32) . The extinction coefficient of the films shows a gradual linear increase with the increase in wavelength in the visible-NIR region of the spectrum.
The porosity of the films was determined from the refractive index of the film and the refractive index (n s ) of SnO 2 (n s 2.006) using Eq. (14) (33) .
https://www.indjst.org/ As depicted in the inset of Figure 6, SnO 2 films possess a porosity of more than 25%. Hence, these films with excellent porous morphology ensure a large solid-to-gas interface due to the high surface-to-volume ratio making it a potential candidate for gas sensing (2,3,34,35) .

Electrical properties
The electrical behaviour of the SnO 2 films was examined by determining the electrical resistivity of the films. Experimental measurements were carried out by dc two-point probe method using silver electrode contacts. SnO 2 films exhibit a high resistivity in the order of 10 -1 Ωcm. Usually, metal oxide thin films prepared by wet chemical method SILAR exhibit high resistivity due to the rough structure and high porosity (14,15) . Due to the small grain size, the density of the grain boundaries in the SnO 2 films may be large. Grain boundaries act as traps for the free charge carriers. These grain boundaries block the transport of charge carriers. Moreover, the SEM images discussed in section 3.2 exposed that the films have a surface morphology with pores. All these factors contributed to the higher electrical resistivity in the films than the reported values of resistivity of 10 -2 Ωcm (4) and 10 -3 -10 -4 Ωcm (11,31) . The prepared SnO 2 films are potential candidates for various thin film resistor applications, which requires high resistivity (36) . Literature study reveals that the film resistivity was reduced by Fluorine doping from 10 − 2 to 10 − 3 Ωcm (4) . The electrical conductivity of the SnO 2 films can be improved by doping with suitable elements like F, Nb, In, Al and Co (7,12,24,28,31) or by annealing (30,37) .

Conclusion
A novel rapid synthesis of polycrystalline SnO 2 thin films was carried at a relatively low temperature 353 K within 20 minutes by SILAR technique using the complexing agent MEA. Films with crystallite size less than 5 nm exhibit a coarse and porous surface morphology. This inventive approach discards the traditional way of post-annealing or sintering to prepare crystalline SnO 2 films. Lattice parameters, cell volume, strain and dislocation density of the SnO 2 films illustrates the quality of the films. The Williamson-Hall technique was utilized for establishing crystallite size and strain of the films. Microstrain developed in the films is of the order of 10 -3 . Transmittance is relatively high in the visible region and the films exhibit nearly 70% transmittance. The optical band gap of the SnO 2 films is 3.65 eV. The refractive index of the films varies from 2.05 to 2.30 in the 450-1900 nm wavelength range. The extinction coefficient of the films shows a gradual linear increase in the entire visible-NIR region of the spectrum. The SnO 2 films exhibit a resistivity in the order of 10 -1 Ωcm. The results obtained from this low-cost method are in par with the results obtained from other sophisticated methods except the electrical conductivity.
The nanostructured SnO 2 films are suitable for various applications such as gas sensors, thin film resistors and dye-sensitized solar cells. Modulation of the electrical and optical behaviour of the films can be explored by doping and annealing. Sensing performance of these films can be further investigated.