Crystal Structure, Lattice Strain, Morphology, and Electrical Properties of SnO2 Nanoparticles Induced by Low Calcination Temperature

)e electrical properties of tin dioxide (SnO2) nanoparticles induced by low calcination temperature were systematically investigated for gas sensing applications.)e precipitation method was used to prepare SnO2 powders, while the sol-gel method was adopted to prepare SnO2 thin films at different calcination temperatures. )e characterization was done by X-ray diffraction, scanning electron microscopy (SEM), and atomic force microscopy (AFM). )e samples were perfectly matched with the rutile tetragonal structure. )e average crystallite sizes of SnO2 powders were 45± 2, 50± 2, 62± 2, and 65± 2 nm at calcination temperatures of 300, 350, 400, and 450°C, respectively. SEM images and AFM topographies showed an increase in particle size and roughness with the rise in calcination temperature. )e dielectric constant decreased with the increase in the frequency of the applied signals but increased on increasing calcination temperature. By using the UV-Vis spectrum, the direct energy bandgaps of SnO2 thin films were found as 4.85, 4.80, 4.75, and 4.10 eV for 300, 350, 400, and 450°C, respectively. Low calcination temperature as 300°C allows smaller crystallite sizes and lower dielectric constants but increases the surface roughness of SnO2, while lattice strain remains independent. )us, low calcination temperatures of SnO2 are promising for electronic devices like gas sensors.


Introduction
Tin dioxide (SnO 2 ) powders and thin films are essential for several applications such as a transparent electrode in panels [1], materials for rechargeable lithium batteries [2], solar cells [3], and gas sensing materials [4]. Due to the remarkable physical and chemical properties, SnO 2 has attracted much attention as a gas sensing material. e interactions between SnO 2 and neighboring gases (carbon monoxide/oxygen) enable this material as an excellent sensor [4]. e efficiency of the SnO 2 gas sensor depends on its particle size, connectivity of atoms, and characteristics of its content. Researchers have shown that the execution of the SnO 2 sensor can be improved by reducing the particle sizes of SnO 2 in the nanometer length scale [4,5]. e sensitivity increases rapidly when the particle size is less than 6 nm, considering as twice the thickness of the Schottky barrier [6].
e SnO 2 powder and thin films can be achieved by several methods, such as sol-gel process [6], chemical vapor deposition [7], a hydrothermal method [8], and sputtering methods [9]. To control the size of SnO 2 particles, there are at least two methods for succeeding. e powder synthesis method is the most common approach requiring standard chemistry apparatus and initial substances. e chemical interaction between reactant and reagent gives the desired product, which appears as colloid or deposit in product solution. Varying concentrations of SnCl 4 and calcination temperatures can adjust the particle size [10], while other research works use spraying tin solution under pyrolysis or hydrolysis situation to obtain nanoparticles [11,12]. e other method for deriving SnO 2 nanoparticle is a thin film growth process using various techniques such as dip coating or sol-gel coating [13,14].
As mentioned in the previous paragraph, the efficiency of the SnO 2 sensor is firmly controlled by the crystal structure and particle sizes of SnO 2 . e precipitation method is a promising technique for controlling SnO 2 crystal structure and particle size because it produces high purity powder, nanoparticle size, and easy composition control and is of low cost [3][4][5]15]. However, only a few studies have examined crystal structure at low calcination temperature, morphology, and electrical properties in SnO 2 powder. Earlier studies reported the features above 400°C calcinations [6,[15][16][17]. e physical and mechanical properties of SnO 2 powders may vary with temperatures. Hence, before using nano-SnO 2 powders, it is crucial to check the above features at low-temperature conditions. Naseem et al. [18] first reported the lattice strain of SnO 2 powder, which can vary due to the solvents used in the synthesis process. [18]. Furthermore, there is no report in the literature about the lattice strain generated between the crystal structure or the lattice mismatch during the phase transformation process.
is lattice strain usually produces the internal electric, which helps in sensing behavior due to the variations in energy bandgap. However, sufficient research is required to strengthen the above statement. e main challenge is how to use the nano-SnO 2 powder and thin films as a gas sensor in industrial uses.
In the present work, we have examined various characteristics of SnO 2 thin films and powders by varying temperature. e crystallite structure, that is, particle size, lattice parameter and lattice strain, morphology, topography and roughness, optical properties, and dielectric property, was explored for achieving a suitable condition to obtain various morphologies of SnO 2 nanoparticle for the application of devices in sensors.  4 . Subsequently, the Sn(OH) 4 solution was filtered, washed, and dried in an oven at 110°C for 24 hours. e reactions may be written as follows:

Materials and Methods
e calcination temperature is an important parameter for determining the electrical property of a substance. Hence, SnO 2 powder was made in a round-shaped plate by a single axis hydraulic press at 20.5 ± 0.5 MPa pressure. en, the samples were set into a tubular furnace by varying the calcination temperatures as 300, 350, 400, and 450°C for three hours with a proportion of 3°C/min heating. e density was calculated using weight and thickness. Figure 1 presents the preparation of SnO 2 nanoparticles.

SnO 2 in Film.
Tin-dichloride-dihydrate (SnCl 2 •2H 2 O) was first dissolved in isopropanol, and then deionized water was added to it at a ratio of 0.6 : 6 : 0.4. While adding the deionized water, continuous stirring is required to start the hydrolysis and polycondensation reactions. Besides, 0.05 ml of acetic acid was used as a catalyst. en, the magnetic stirrer was used for 3 h at 25°C to make the sol completely homogeneous and transparent, as reported by Terrier et al. [21]. e ITO-coated glass substrate [13], which is available in the market, was cut into equal pieces with a similar dimension of 1 × 1 inch 2 . e substrates were washed with deionized water and acetone to remove dirt. To overcome further impurities attached to the ITO substrate, an ultrasonicator was used to rinse with methanol and deionized water alternatively thrice. Substrates were then immersed into an earlier prepared solution for 5 min and pulled out with a fixed velocity with 3.0 mm/min, and the same procedure repeated ten times to achieve a desired thickness of the film.

Characterization.
e crystal structures were obtained by X-ray diffraction (XRD, Bruker D8 Advance with Eulerian Cradle; CuKα1 with λ � 1.54060Å, Spain) within the diffraction range of 2θ � 20°to 80° [7,8]. e diffraction data were recorded at a 3°min −1 scan rate for the analyses of crystal phases [22]. e crystalline size (β crystalline ) and lattice strain of the sample (β strain ) n are related to the width (β r ) [23]: Here, (β crystalline )values were estimated from the "Scherrer-Gottingen equation." (β crystalline ) can be calculated by considering Bragg reflections with diffraction angle (2θ) [24]: where K is a constant and considered as dimensionless shape factor (K � 0.89); λ is a wavelength of CuKα radiation. Hence, the lattice strain can be represented as follows: where η denotes the strain of the sample. us, the total width of the diffraction peak becomes 2 Advances in Materials Science and Engineering In the graph, β r cos θ against sin θ, one can obtain the lattice strain of the sample η from the slope and the crystallite size (L) from the intercept (Kλ/L).
Scanning electron microscopy (SEM, Hitachi S-3400N, Japan) with an energy-dispersive X-ray spectrometer (EDX, Oxford instrument, UK) was used to investigate the arrangement of atoms in the crystal, particle size, and chemical compositions of SnO 2 powders. Gold and carbon were coated onto samples for SEM and EDX analysis, respectively. e topography and crystallite size of SnO 2 powders were investigated by atomic force microscopy (AFM, Nanosurf, Nanosurf Easy Scan 2, Switzerland) at room temperature. e principle and experimental setup of the AFM is described in the literature [25]. A commercial tip cantilever system (Scout 350, Nunano, UK) with spring constant of k � 42 N/m and tip apex radius less than 10 nm was used. Measurements were carried out in noncontact mode acquiring 256 pixels × 256 pixels images with a rate of scanning 2 to 5 Hz. e permittivity was calculated from the measured capacitance by using a parallel plate method. e vector components of capacitance (C) and dissipation (D) were measured by the instrument. Finally, a computer program was used to calculate the relative permittivity and tangential loss as described from the following equation [26]: Here, t denotes the thickness and A signifies the area of SnO 2 samples.
A double beam UV-Vis Spectrophotometer (Biochrom Libra S80, UK) was used to perform the optical properties of SnO 2 films at room temperature with wavelength range 200-800 nm. From the reflection R(λ) and transmission T(λ) data, the absorption coefficient (α) was calculated using the following equation [27]: Here, t is the thickness of the film. For direct energy bandgap, the above equation can be written as where E g is the optical bandgap and h] is the energy of a photon. A plot was generated earlier (αh]) 2 versus h] for films at different calcination temperatures [28].

Effect of Calcination
Temperature on the Crystal Structure of SnO 2 Powder. At low calcination temperature, the measured XRD pattern of the product exhibited a very rough signal as several stray peaks appeared over the scan angles. However, some important broad peaks of SnO 2 were strong enough to be identified. Figure 2 presents the XRD patterns obtained from the powder SnO 2 samples at various calcined temperatures. e sharpness and an increase in peak intensity are attributed to the rise in calcination temperatures by crystallinity [16,17]. e strong intensity was observed along (110) plane, while comparatively weak intensity was detected along (101)   Advances in Materials Science and Engineering 3 than 300°C and 350°C. e crystallinity of the 300°C regimes indicated an amorphous structure as low intensity and broad peak attribution were found. Since the calcination temperature was increased, the intensity and width of XRD peaks became high and narrow, respectively, suggesting that powder SnO 2 obtained enough energy to form a perfect crystalline form.
Using equation (7) and Figures 3(a)-3(d), the average sizes of nanocrystalline SnO 2 powder were obtained, which could be 45 ± 2, 50 ± 2, 62 ± 2, and 65 ± 2 nm. e data in Table 1 manifest a linear relationship betweenβ r cos θ and sin θ. From the experiments, the corresponding lattice strains became 0.0087, 0.0102, 0.0116, and 0.0084 while treated at 300, 350, 400, and 450°C, respectively. Table 1 presents the crystallite size of the sample, which was increased by the rise in calcination temperature. Hence, the continuing growth process was also found in the present research, as well as in the literature [10,11]. Besides, a rise of the crystalline size of about 5 nm was observed after calcination at 400°C for 3 hours, which explains a small growth trend under such conditions. e XRD data revealed the polycrystalline nature of SnO 2 powder, which was fitting well in the tetragonal crystal with a space group P42/mnm. e corresponding lattice parameters were found as a � b � 4.73 ± 0.04Å, c � 3.18 ± 0.04Å [2]. e lattice strains were 0.0102 and 0.0116, with 350°C and 450°C, respectively. e strength increases with the rise in temperature, which signifies that the structure might reorient due to the process of phase transformation. e internal electric field was produced between the grains of SnO 2 powder and enhanced the electrical property of the SnO 2 gas sensor. SnO 2 Powder. Figure 4 depicts the SEM micrographs of SnO 2 powder calcined at different temperatures. At low temperature (300°C), the distribution of SnO 2 particles is inhomogeneous, as shown in Figure 4(a), while raising the temperature (400°C) causes an improvement in particle distribution. Besides, Figures 4(b)-4(c) reveal that the crystallite sizes of each sample are uniform, depending on the calcination temperature. It is to be noted that a low calcination temperature takes more energy for crystal growth than high calcination temperature, as reported in the literature [12,13,29]. However, the SEM images show that high calcination temperature (400°C and 450°C) helps to form more crystalline states than the lower one (300°C and 350°C); this finding matches with the XRD result [6].  Table 1.

Effect of Temperature Calcinations on the Morphology of
e aggregation of the 200-400 nm particles only occurred in the samples prepared at the lower annealing temperature. ese nonhomogeneous particles are usually observed when the annealing temperature of a sample was decreased to 300°C-350°C [30]. is is because the lower annealing temperature needs more energy growth than a higher annealing temperature. According to the fact that the SnO 2 are crystallizes into the polymorphs, the observed 200-400 nm particles of SnO 2 ( Figure 5(b)) are composed of rutile type (P42/mnm) and pyrite type (Pa3) phases. e purification of SnO 2 powder investigated by the EDX indicated the atomic weight ratio of Sn by the increase in calcination temperature. At 300°C, the chloride atom left from the precipitation process and dominated over the synthesized crystal (Figures 6(a)). However, the Sn atom was found as a larger weight percentage in other temperatures (Figures 6(b)), as shown in Table 2. Figure 7 depicts a series of AFM topographs of SnO 2 powder with different calcination temperatures. e surface roughness and the crystallite size (40-100 nm) increase by rising calcination temperatures as the atoms obtain enough energy at higher temperatures.
ere is a probability of recrystallization at higher temperatures as atoms often change their position. is finding is similar to the results reported in the literature [31,32]. e density and homogeneity of the SnO 2 powders were improved after calcination, showing a strong agreement with the results obtained from XRD. Not only does calcination induce an apparent increase in the size of crystal but also it changes the phase content, which is the ratio between rutile (P42/mnm) and pyrite (Pa3) type phases. However, at a low calcination temperature as low as 300°C-450°C, the phase ratio was evaluated from the diffractograms and found to be quite stable (the results not shown here). e size of the crystals increases by increasing the temperature of calcination. However, it is recommended to have a small particle size for gas sensors as a higher surfaceto-bulk volume ratio increases the adsorption of gases at the sensor surfaces, leading to enhanced sensitivity. e results shown in Figures 2-7 indicate that the crystallite size of SnO 2 powders is improved in the range of 40-100 nm after calcination. Hence, the present samples are promising for sensor applications.

Effect of Temperature Calcinations on the Dielectric Constants of SnO 2 Powder.
e electrical features of SnO 2 powders were investigated by measuring the dielectric constant at different calcination temperatures. Figure 8 presents the behavior of dielectric properties with the effect of frequency at normal temperatures. e dielectric measurement of nanoparticles is a difficult task, and the present results could influence the apparent porosity. Pressing the SnO 2 samples at 30.5 ± 0.5 MPa could eliminate the porosity effect of the dielectric measurement. During the analysis, several LC resonances were noticed on the graphs between dielectric constant and frequency. e resonance might come from the inductance of the wires but was removed before calculation. e dielectric constant (ε′) decreases abruptly at lower frequencies while achieving a saturation at frequencies above 10 MHz. e inset of Figure 8 shows the saturated ε′ at high frequencies, illustrating ε′ as almost frequency independent and can be explained by the polarization mechanism. e dipole moment changes by varying applied electric field at low frequency, resulting in polarization. But dipole moment did not follow specifically by changing the applied electric field, resulting in a decrease in ε′ at a higher frequency. e values of ε′ obtained in the present research are much lower than the reported values [23,29], attributing to the inhomogeneity of the dielectric structure. e inhomogeneity could occur due to the presence of   Figure 6: EDX spectra data for SnO 2 at calcination temperature of (a) 300°C and (b) 400°C. grain structure, porosity, crystal defects, and stoichiometry. Figure 9 presents the plot of dielectric losses (ε″) at different calcination temperatures and frequencies. At higher frequency, ε″ decreases and, thus, polarization occur. At 400 Hz, ε″ becomes maximum, indicating the dissipation of samples and forms like loose materials while using in an external electric field.

Effect of Temperature on the Optical Bandgap of SnO 2
Films. Figure 10 presents an optical absorbance of SnO 2 nanoparticles at different temperatures of calcination. In the spectra, absorption is high at the lower wavelength region, attributing two sets of wavelengths. A sharp increase in absorption manifests in the first region (200 nm ≤ λ ≤ 250 nm), while, in the second region (250 nm ≤ λ ≤ 290 nm), an abrupt fall in absorption occurs. e average absorbance in the visible region lies within 25-30% by varying the calcination temperatures, while maximum absorbance comes to 38.5% at 450°C. By reducing the calcination temperature, the UV-visible absorbance shifts towards a smaller wavelength due to the decrease in particle size. Furthermore, the smaller sizes of particles can absorb at shorter wavelengths, and, thereby, the bandgap increases, showing a good agreement with the present SEM and XRD data.   8 Advances in Materials Science and Engineering Figure 11 presents the plot of energy of the optical bandgap at different calcination temperatures. e direct bandgap energy (E d ) values can be found from the intercepts with the energy axis. e bandgaps are 4.85, 4.80, 4.75, and 4.10 eV for 300°C, 350°C, 400°C, and 450°C, respectively. Notably, E d values calculated from the present analysis are higher than those found in the reports [5,16,18].
ese larger values are achieved due to the variation of the stoichiometry of the concentration of point defects associated with SnO 2 films. Furthermore, E d decreases with the increase in crystallite size, although independent of the lattice strain. A similar trend has been observed in earlier reports [33].

Conclusions
We successfully prepared nano-SnO 2 particles by the precipitated method. XRD analysis revealed a tetragonal rutile structure with P42/mnm (136) phase group from the synthesized compounds. e average crystalline size increases from 45 ± 2, 50 ± 2, 62 ± 2 to 65 ± 2 nm due to the increase in calcination temperature from 300, 350, 400, to 450°C, respectively. ese results also show the persistence of lattice strain, which is independent of calcination temperature. SEM images show that the average particle size and grain size increased while increasing the calcination temperature. EDX analysis of the samples showed the existence of tin and oxygen in the pure sample. AFM images showed that the rise in calcination temperatures contributes to high surface roughness and large crystallite size. e dielectric constant was much lower than the expected value. e energy bandgap of SnO 2 thin films was independent of their lattice strain but decreased with an increase in their crystallite size. Lattice strain was ambiguous and needed further investigation. However, the morphology of SnO 2 powder can be altered in terms of particle size and crystal structure by different calcination temperatures, which could improve the sensitivity of the sensor. Hence, the present research caused by shallow calcination temperature should be carried out for future direction.

Data Availability
e data used to support the finding of this study are included within the article. (αhv) 2 (eV.cm -1 ) 2 Figure 11: e energy bandgaps of SnO 2 films with different calcination temperatures (Touc plot of (αh]) 2 with photon energy in SnO 2 thin film).

Conflicts of Interest
Advances in Materials Science and Engineering 9