Characterization and optical gas sensing properties of BaSnO3 synthesized by novel technique: flame spray pyrolysis

Barium stannate (BaSnO3) particles were synthesized using a one-step flame spray pyrolysis (FSP) method. The fabricated ceramic powders were investigated in terms of the structural, morphological, and optical properties by using X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), zeta particle size analyzer, UV–Visible spectroscopy (UV–Vis) and photoluminescence spectroscopy (PL). The XRD results showed the structure of BaSnO3 crystals has been obtained when the powders were exposed at high temperatures, specifically at 1200 °C. The synthesized particles in the submicron size in a range of 70–980 nm were produced. The optical bandgap value of the synthesized crystals was calculated using reflectance spectra with the Kubelka–Munk method and found as 3.14 eV. When the powders were excited at 375 nm, they exhibited emission bands in the visible and near-infrared region (NIR) of the electromagnetic spectrum. In this study, the intensity- and decay time-based gas sensing properties of nanoscale BaSnO3 embedded in ethyl cellulose (EC) thin films when exposed to the vapors of different solvents were also measured. The optical sensitivities (I-I0/I0) of the sensing materials for the NH3, acetone, and EtOH solvent vapors were calculated as 0.835, 0.672, and 0.521, respectively.


Introduction
The perovskite oxide structures with a chemical formula of ABO 3 have gained great attention for the applications of gas sensors, solid oxide fuel cells, catalysts, etc. [1,2]. In the chemical formula of perovskite oxides, 'A' and 'B' are two metal ions of various sizes, and O is an oxygen element, which bonds to both cations. Alkaline or rare earth elements are occupied at the position of the corners (A), transition metals (3d, 4d, and 5d) are placed on the central site (B), and oxygen element is located at surface centers. Sometimes, as a result of the distortions, the perovskite structures that have a cubic crystalline phase can modify to orthorhombic or tetragonal phase [3]. However, the difference in the symmetries is based on a mismatch of alkaline cations because of the smaller ionic radius to the cubic-octahedral cavity in the octahedral lattice. Since the electrical and chemical properties of these materials may change, it should not be forgotten that electronegativity and doped elementals have an important effect on these properties and should be selected carefully accordingly [4].
The barium stannate (BaSnO 3 ) crystals formed as a cubic perovskite-type belong to the group of alkaline earth stannates (MSnO 3 , where M = Ba, Sr, Ca, Pb) and n-type semiconductors [5]. BaSnO 3 has a wide bandgap value of 3.4 eV [6] and has many applications in the field of optoelectronic devices [7], gas sensors [8], photocatalytic studies [9]. BaSnO 3 crystals are used as a gas sensor, which is sensitive to a variety of gases such as O 2 [10], CO 2 [11], CO [12], SO 2 [13], CH 4 [14], and humidity [15]. Cerdà and coworkers have investigated the electrical characteristics of BaSnO 3 produced using a wet chemical route method as a function of both the temperature and the gas concentration [16]. It is also used to prepare electrical conductor materials because of its dielectric features [17]. Bévillon et al. defined that barium stannate ceramics as a protonic conductor, because of including trivalent metals and the diffusion of protons once H 2 O dissociates in the oxygen vacancies [18]. BaSnO 3 crystals have been used in photocatalytic degradation applications. The synthesized BaSnO 3 particles are used as a suitable material for photocatalytic applications such as removal of Eriochrome Black T under the lightning of UV radiation by Moshtaghi and coworkers [19]. Manjunath et al. designed the La-doped BaSnO 3 sensor with a gas response of concentration of * 65 and * 29 (at 50 ppm) for ammonia and formaldehyde gases as based on electrical signals measurement [20]. Tao et al. presented that BaSnO 3 gas sensor prepared by low temperature precipitation method exhibits low response to LPG, CO, H 2 and petrol, and high gas response and good selectivity to ethanol [21]. The gas detection behavior of BaSnO 3 has been investigated to reduce and oxidize gases such as butane, ethanol, CO and NO 2 from 5 to 50 ppm concentration levels by Kumar and coworkers [22]. According to the literature, many metal oxide-based gas sensor studies are based on the electrical measurement method. On the other hand, the advantage in luminescence-based detection is the absence of electrical contact formation between nanostructures and the resulting metallic impurities. However, the effect of adsorbed gases on the photoluminescence of metal oxide powders has not been studied sufficiently yet.
Synthesis of pure mixed metal oxide powders such as BaSnO 3 is quite difficult. A common synthesis method is to mix barium and tin ions at the molecular level in a position closely together at high temperatures. There are many different techniques applied for the synthesis of barium stannate such as sol-gel [23], hydrothermal [24], modified combustion technique [25], the solid-state ceramic method [26] and wet chemical synthesis method [27], and one-step flame spray pyrolysis (FSP) technique. For the synthesizing of nanoparticles, the FSP is the most favored technique because of reducing the synthesis time and the increased reaction rate. Some of the advantages of the FSP method are the ability to disperse the precursor directly in the fuel, the simplicity of spraying the precursor into the flame, and highvelocity aerosol formation. For this reason, the FSP has been applied many times in the production of nanoscale metal oxides and complex metal oxides. Due to the high-temperature environment of the flame (up to 2500°C, depending on raw materials and flame conditions), product nanoparticles are typically fully oxidized and crystalline. Thus, postsynthesis heat treatment steps are typically not required and the nanopowders can directly be used for further processing. However, in the production of complex metal oxides, a heat treatment process may sometimes be required. In this report, the pure perovskite-type BaSnO 3 particles have been synthesized utilizing the FSP method and investigated in terms of structural, morphological, and optical properties.
When the sensing particles were excited at 375 nm, they exhibited emission bands in the visible area, and also, a strong peak centered at 944 nm in the nearinfrared region (NIR). Also, in this study, the photoluminescence-based optical gas sensing properties of the BaSnO 3 nanoparticles embedded in ethyl cellulose thin films toward the vapors of ethanol, acetone, and ammonia were presented.
2 Experimental studies

Materials
All chemicals and solvents were used without any further purification. Barium acetate (Ba(CH 3 COO) 2 ) and Tin (IV) chloride pentahydrate (SnCl 4 9 5H 2 O) were purchased from Sigma-Aldrich. Ethylene glycol and citric acid were also provided from Sigma-Aldrich. Ethyl cellulose (EC) and tetrahydrofuran (THF) were supplied from also Sigma-Aldrich. Ethanol (EtOH), acetone, and ammonia (NH 3 ) were supplied from Fluka. Methane (CH 4 ) and oxygen (O 2 ) gases were supplied from Tinsa Gas Company.

Instrument
Determination of phase structure of produced BaSnO 3 (BSO) powders was carried out by Thermo Scientific ARL X-ray diffractometer (XRD) which operates with 45 kV voltage and 44 mA current settings and uses Cu-Ka radiation (1.5405 Å ). X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha) with a monochromatic Al-Ka (1486.7 eV) X-ray source and a beam size of 400 nm diameter was carried out to determine elemental composition and surface chemistry for all samples. The FTIR spectra were measured to identify the powder structure in the range 500 and 4000 cm -1 wave number with the Thermo-Scientific NICOLET IS10 spectrophotometer equipped with the attenuated total reflectance (ATR) apparatus. Microstructure images of particles were captured at different magnifications by using a scanning electron microscope (SEM, Zeiss Sigma 300 VP) with 20 kV accelerating voltage. Particle size analysis of all of the synthesized composites was examined by using a Malvern Zeta Sizer ZS90. The reflectance spectra of the samples were measured using Thermo Scientific Evolution 600 spectrophotometer. The steady-state photoluminescence (PL) emission/excitation measurements were studied by using a red-sensitive photomultiplier tube-equipped spectrofluorometer system (Edinburgh Instruments (UK). The instrument was also equipped with a standard 15 W xenon lamp for steady-state measurements.

Flame spray pyrolysis process
The general processing of the flame spray pyrolysis system is demonstrated in Fig. 1. In the flame spray pyrolysis technique, a transparent metal ion solution is sprayed as a thin haze into the center of flame which is fuel/oxygen ratio of methane (1.5 L/min)/oxygen (3.0 L/min) through a syringe pump with feed rate of 5 mL/min. Then, tiny droplets are created while the solution burns within the flame. The transformation of the salt into the metal-oxide structure occur upon the pyrolysis reaction. After that, metal oxide atoms aggregate into nanoscale particles and these collected on a cellulosic filter by using a vacuum pump.

Synthesis of BaSnO 3
In this study, BSO powders were synthesized by flame spray pyrolysis (FSP). Ba(CH 3 COO) 2 and SnCl 4 .5H 2 O salts were used as precursors. At first, ethylene glycol and citric acid were dissolved as a 4:1 ratio at 70°C, and then 0.2 M Ba(CH 3 COO) 2 and SnCl 4 .5H 2 O were added to this solution. The solution was stirred until a transparent mixture was obtained. Figure 1 shows the experimental design of the FSP equipment. The resulted solution was fed to flame spray pyrolysis equipment (Tethis Nps10) using methane (CH 4 ) and oxygen (O 2 ) gases to obtain BSO particles. The liquid precursor solution was fed into flame by a syringe pump with a feed rate of 5 mL/ min and dispersed by oxygen to form the fine spray. The pressure was conserved constant at 2 bars at the nozzle by adjusting the orifice gap. This premixed CH 4 /O 2 supporting flame was conserved constant at a flow rate of 5 L/min and fuel/oxygen ratio of methane (1.5 L/min)/oxygen (3 L/min). The synthesized powders by FSP were picked using a watercooled, stainless-steel filter housing supporting a glass microfiber filter (Whatman GF6, 257 mm in diameter), with exhaust gases drawn by a vacuum pump. To investigate the changing in phase structure of BSO particles, all of the powders were annealed at different temperatures as 600°C, 800°C, 1000°C, and 1200°C, respectively. In order to facilitate the interpretation of the results, BSO particles synthesized by FSP were encoded as BSO_FSP.

Preparation of BSO_FSP-based thinfilm sensors
Polymeric materials are constantly used as matrix materials in optical sensor designs. In the production of the sensing slides, ethyl cellulose was chosen as support material because of the high diffusibility of the solvent vapors. The EC matrix has not any intrinsic color/luminescence, and it is optically transparent in the spectral range where measurements are being performed. Therefore, the BSO_FSP crystal particles were embedded in the EC matrix which allows free diffusion of the gas molecules that might react at the surface of the crystalline molecule. In this work, the sensing material was prepared by mixing 100 mg of EC and 0.25 mg of BSO_FSP in 2.5 mL of THF. All of the constituents were mixed under magnetic stirring. The prepared polymeric mixture was spread onto polyester support (Mylar TM type) by knife coating technique. Tencor Alpha Step 500 Profilometer was used to measure thicknesses of the coated thin films. The average thickness of the films was found to be 7.25 ± 0.13 lm (n = 5).

Solvent vapors performance tests
The solvent vapor performance tests of the BSO_FSP nanoparticles were performed after exposure to various solvent vapors like ethanol (EtOH), acetone, and ammonia (NH 3 ) of the same test material in a closed desiccator. This group of tests was named 'solvent vapors tests.' We exposed the sensor slides to the different concentrations of solvent vapors in a closed desiccator with an internal volume of 5 L by placing 20 mL of solvent (contained in the 50 mL beaker). The vapors were obtained by evaporation of different solvents at room temperature. The mean laboratory temperature was 20°C (± 0.6°C) throughout the experiments. Concentrations of the solvent vapors were calculated using their partial vapor pressure at 20°C (EtOH: 5.95 kPa, acetone: 25.00 kPa, and NH 3 (25% v/v): 40.32 kPa). The partial pressures were also converted to per million part (ppm) for all of the solvent vapors were found to be 2.24, 11.90, and 5.60 (mg/20 ml) for the EtOH, acetone, and NH 3 , respectively. Utilization of the BSO_ FSP crystals in EC in the form of thin films resulted in many advantages such as enhanced sensitivity and improvement in all optical gas sensing dynamics. Sensing characteristics of the offered composites were tested after exposure to vapors of EtOH, acetone, and NH 3 , respectively. The structural changes of the particles produced by the FSP method at different calcination temperatures were examined by XRD, and the related patterns of the BSO particles are shown in Fig. 2. When the phase structure of the material was examined after the production, an amorphous structure was observed. At temperatures of 600 and 800°C, a partially crystalline state began to form, but no full crystalline structure was formed. At temperatures of 1000 and 1200°C, the material was crystalline, but at 1000°C, besides the BaSnO 3 phase with cubic perovskite structure, the SnO 2 phase was also determined. At 1200°C, the material was found to have a completely BaSnO 3 structure. All the peaks agree with the reported JCPDS data (Card No. 15-780). These results are consistent with the studies in the literature [24,28,29].
The structural and lattice parameters of the BSO material were calculated with the Rietveld refinement using the Maud 2.71 V software, and detailed results are given in Table 1. The lattice parameters of the structure were compared with the JCPDS file (No. 15-0780) [30] for BaSnO 3 , and very close values were observed. Based on the Rietveld analysis, the structural parameters of BaSnO 3 were found to be a cubic perovskite structure with space group #221 (Pm3m) and lattice parameter 4.1 Å . The crystallite sizes were estimated from the broadening of the major (110) peaks using Debye-Scherrer's formula (D = k.k/b.cosh) [31], where D is the crystallite size (nm), k Cu-= 0.15406 nm, b is the full-width at half-maximum (FWHM) of the peak in radians corrected from instrumental broadening and h is the peak angle [32]. k is the so-called Scherrer constant. k depends on the crystallite shape and the size distribution, indices of the diffraction line, and the actual definition used for b whether FWHM or integral breadth. k can have values anywhere from 0.62 and 2.08. In this study, k = 0.94 value for FWHM of spherical crystals with cubic symmetry was used to estimate the crystallite size D [33].The crystallite size for the major (110) reflection of the BSO sample calcined at 1200°C was determined to be about 80 nm.

Elemental analysis (XPS)
The sample annealed at 1200°C was analyzed by XPS for identification of the surface's chemical states and elements whose binding energies (BE) and weight concentration (wt%) are listed in Table 2. The binding energies of 777.75 eV, 485.23 eV, 529.32 eV, and 284.93 eV are corresponding to Ba3d, Sn3d, O1s, and C1s, respectively, which is shown in Fig. 3a. The C1s peak at 284.93 eV is owing to the carbon contamination which comes from carbon tape used for binding powders to the holder for analyses [34,35]. Ba3d and Sn3d are composed of two doublet peaks which are termed as d 3/2 and d 5/2 while O1s and C1s are present in single and/or deconvoluted peaks. Ba3d 3/2 and Ba3d 5/2 are the doublet peaks of Ba3d and peak positions are 792.58 eV and 777.28 eV, respectively, which are depicted in Fig. 3b. The difference between these peaks is 15.30 eV which is corresponding with the Ba 2? state [34,36]. Sn3d consists of doublet peaks of Sn3d 5/2 (485.08 eV) and Sn3d 3/2 (493.48 eV) which are shown in Fig. 3c, and peak differences between these are 8.4 eV which is related to Sn 4? state [34,36,37]. The deconvoluted O1s core-level spectra are shown in Fig. 3d. The peak of 529.32 eV is related to lattice oxygen while the peak of 532.05 eV is related to surface hydroxyl groups [32,38,39]. XPS survey and core-level spectra confirm that Ba, Sn, and O are the valence states of 2 ? , 4 ? , and 2-that form cubic perovskite BaSnO 3 and inhibit the presence of any other metal-oxygen composites [40].

FTIR measurements
To get information on the evolution of the functional groups and confirm the composition during the crystallization of the synthesized powders, the annealed powders at different temperatures from each synthesis method were characterized using FTIR spectroscopy. Figure 4 shows FTIR spectra of BSO_FSP particles as before annealed and calcined at from 600 to 1200°C. As seen from this graph, the broad bands at approximately 3400 cm -1 and 1600 cm -1 can be attributed to hydroxide (H-OH) vibrational and flexing mode, respectively. The band occurring approximately at 1420-1430 cm -1 is due to the vibration of the residual organic C-H groups. On the other hand, the bands at 611 and 623 cm -1 correspond to the Sn-O. All the bands of these spectra are in good agreement with the literature [28,41].

Particle morphology and size distribution measurements
SEM images and particle size measurements were performed to look into the morphology, size distribution of the BSO_FSP particles calcined at 1200°C, and the results are shown in Fig. 5. The annealed particles at 1200°C with a dimension of between 400-1000 nm show a polyhedral-shape morphology, and the average size of the powders is calculated as approximately 700 nm. According to the obtained  results, while the temperature is increasing to 1200°C, the mean grain size of BSO_FSP powders grows from 70 to 980 nm and the size distribution of particles becomes wider.

Optical measurements
The reflectance spectra (see Fig. 6) measurements were performed by using UV-Vis DRS to calculate the optical bandgap of BSO_FSP at 1200°C with the following equation (see Eq. 1) [42]: where a is absorption coefficient, h is Planck's constant, m is the frequency of incident photons, A is the energy-independent constant, E g is bandgap and n is the index determining the type of transitions and could be values of 2, 3, 1/2, and 3/2 for indirect allowed, indirect forbidden, direct allowed, and indirect forbidden transition, respectively [43]. Kubelka-Munk theory is commonly used to analyze the optical bandgap for materials that weakly absorb incident photons and function was applied to convert reflectance values to absorbance and determine optical bandgap [44]. The optical bandgap of the sample annealed at 1200°C was determined with the help of the following Kubelka-Munk equation (see Eq. 2) [42]: where F(R) is the Kubelka-Munk function which is related to a as a = F(R)/d (d is the thickness), and R is the reflectance. The results show that BSO_FSP annealed at 1200°C has a 3.13 eV optical bandgap and it is in agreement with the literature [24,34,42]. The photoluminescence spectra that include the optical transitions of BaSnO 3 are shown in Fig. 7. The emission and excitation spectra were measured at room temperature. When excited at 375 nm, the particles yielded sharp emission bands centered at 444, 485, 505, and 560 nm. The broad line at 444 is attributed to the transition of 3 P 0 0 -3 D 1 of barium.
The medium-intense emission peak at 485 nm is due to the transitions of 3 P 0 1 -1 D 2 of tin. The peak observed at 505 nm is referred to as the transitions of 3 P 2 0 -3 D 3 of barium. The strong line at 560 nm is attributed to the transitions 3 P 0 0 -3 D 1 of barium [25]. Also, the crystals have strong emission band maxima at 944 nm in the near-infrared (NIR) region. A broad and strong near-infrared luminescence centered at 944 nm is attributed to the BaSnO 3 structure [45].

Optical gas sensing mechanism
In polycrystalline oxides, oxygen vacancies are known to be the most common defects and form the donor level [46]. BaSnO 3 crystals are produced by the flame spray pyrolysis method, and there should be the existence of oxygen vacancies because of rapid evaporation and oxidation processes. The emission peaks in the green region of the spectrum are attributed to the electron transition from the donor level formed by oxygen vacancies. Also, oxygen vacancies compose defect levels that act as traps that cause additional defect peaks of luminescence in the NIR region [47]. The main factor that enhances the gas sensitivity is the creation of surface-related defects in the nanoparticles, which cause a broad absorption in the spectrum, during the formation of the composite [48].
As can be seen in the previous studies [10][11][12][13][14][15], the polyhedral-shape structure encourages us to think that the large specific surface area of BaSnO 3 crystals improves gas sensing performance. Herein, we investigated the gas sensing properties of the BSO_FSP when exposing to the different solvent vapors. When excited at 375 nm, the split emission peaks appeared at 500 and 560 nm in the thin film form, respectively. The exposure time of the solvent vapors to BSO_FSP thin films was 60 min. at laboratory conditions. When the prepared thin films were exposed to the vapors of ethanol (EtOH), acetone, and ammonia (NH 3 ), both of the excitation and emission spectra of the BSO_FSP exhibited significant signal drop (see Fig. 8). Results were calculated as relative fluorescence changes using an algorithm (I -I 0 )/I 0 for the y-axis where I 0 and I are the intensities of fluorescence in the absence and the presence of the quencher, respectively. The gas sensitivities (I-I 0 /I 0 ) of the BSO_FSP-based sensing materials were calculated as 0.835, 0.672, and 0.521, when exposed to the vapors of NH 3 , acetone, and EtOH, respectively. Herein, the observed signal drops in the emissionbased intensities of the BSO_FSP embedded in the polymeric matrix may be attributed to the interaction between oxygen vacancies in the surface area and gas molecules. Furthermore, it is thought that the interactions between the oxygen in the lattice structure of BaSnO 3 and the vapors of three different solvents have increased as a result of the increase in the surface area of BaSnO 3 nanoparticles. The first potential possibility behind the high ammonia sensitivity of BSO_FSP may be due to the existence of an unshared single pair of electrons in the NH 3 structure. The second probability is that an ammonia molecule which has a smaller kinetic diameter (0.36 nm) may be diffused into the polymeric EC membrane easily and contact with chemisorbed oxygen vacancies quickly than the other gas molecules. This interaction can be attributed to significant signal drops in the intensity-based measurement [20,49,50]. Fig. 8 Intensity-based response of the BSO_FSP powder embedded in EC thin films exposed to solvent vapors as NH 3 , acetone, and EtOH Fig. 9 Fitted decay time spectra of the BSO_FSP powder embedded in EC thin films exposed to solvent vapors as NH 3 , acetone, and EtOH 3.7 Decay time measurements of BaSnO 3 embedded EC thin film exposed to EtOH, acetone, and NH 3

vapors
In this study, the microsecond timescale decay time measurements of the BSO_FSP powder embedded in EC thin-film matrix when exposed to EtOH, acetone, and NH 3 were also investigated (see Fig. 9). The decay kinetics also give us valuable information about the mechanism of interaction between the BSO_FSP powders and various solvent vapors. The decay curves of all BSO_FSP-based thin films were well fitted by mono-exponential decay function when excited at 375 nm. Table 3 reveals the variation of the decay times in different vapors, EtOH, acetone, and NH 3 . As seen from Table 3, the decay time values of BSO_FSP powder in the EC matrix decreased from 11.11 ls to 11.03 ls, 10.62 ls, and 10.29 ls levels upon exposure to the vapors of EtOH, acetone, and NH 3 , respectively. The surface-adsorbed oxygen species and the enhancement in surface area play a very important role in the gas sensing process [51]. However, the similar decrease in decay time values as in intensity-based measurements can be attributed to the interaction between oxygen in the lattice structure of BaSnO 3 and vapors of three different solvents.

Conclusion
BaSnO 3 particles were successfully synthesized, and the cubic phase structure was obtained at 1200°C. As a result of the elemental analysis, no element was observed except the elements existing in the structure, and the structure was produced in the requested stoichiometric ratio. After morphological investigations, it was observed that the particles were polyhedral and the particle size was submicron. When the optical properties of the BaSnO 3 structure were examined, it was found to be a semiconductor material. BaSnO 3 particles exhibited maximum emission bands at 444, 485, 505, and 560 nm in the visible area, and also, a strong peak centered at 944 nm in the near-infrared region (NIR) under excitation at 375 nm. The intensity-and decay timebased gas sensitivities, which are one of the characteristics of perovskite BaSnO 3 structure, were studied optically, and it was found to respond well to ethanol, acetone, and ammonia gas vapors. In the light of this information, it was determined that the synthesized material can be used in optoelectronic and gas sensor applications.