A Novel Palm Sugar-Mediated Hydrothermal Synthesis of Spherical Tin Oxide for Enhanced SO2 Gas Detection

Sulfur dioxide (SO2), a hazardous gas resulting from combustion and natural reactions, poses environmental and health risks. This study presents a novel approach to synthesize tin oxide (SnO2) using palm sugar-mediated hydrothermal methods for enhanced SO2 gas sensing. The simplicity and cost-effectiveness of the method are highlighted, addressing challenges posed by complex and resource-intensive conventional methods. Spherical SnO2 particles were successfully synthesized and characterized using SEM and XRD techniques. From the SEM image, it was known that SnO2 has spherical morphologies and is expected to agglomerate after being calcined. XRD analysis shows that SnO2 has a rutile tetragonal phase structure. The synthesized SnO2 demonstrated favorable gas sensing properties when exposed to SO2, exhibiting elevated response at increased temperatures and a linear relationship between response and gas concentration. The results indicate the potential of this method for effective and efficient SO2 detection.


Introduction
Sulfur dioxide (SO2), a colorless hazardous gas, is generated through the combustion of sulfurcontaining substances and natural chemical reactions.SO2 can contribute to the formation of acid rain due to its capability to react with atmospheric oxygen, posing a threat to ecological life [1]- [4].However, its implications extend even further, including human health concerns.With a short-term human tolerance threshold of approximately 5 ppm and a long-term exposure limit of 2 ppm, there exists the potential for it to trigger severe respiratory, cardiovascular ailments, and even lung cancer [5]- [7].Consequently, manufacturing effective SO2 detection systems, particularly for low concentrations, emerges as a pivotal task.
Chromatography and spectroscopy were extensively utilized for monitoring SO2 gas.However, these methods are unfavorable due to complex operation, high costs, and large dimensions, which compromise their effectiveness in detecting SO2 [3].Recently metal oxide semiconductor gain lot attention as SO2 gas sensing candidate due to its simplicity, cost-effectiveness, and controllability [8]- [10].Among others metal oxide materials exploited for SO2 sensing, tin oxide (SnO2) has emerged as a noteworthy candidate [11].SnO2 is an n-type semiconductor oxide with a significant energy gap (Eg = 3.6 eV).Its broad bandgap and robust electrical conductivity leads to superior sensitivity and the capability to detect volatile gases, even in low concentration [8], [12].Several methods, such as electrospinning [13], [14], thermal evaporation [15], [16], sol-gel [17]- [19], and CVD [20], [21], have been employed for synthesizing SnO2.However, these methods are complex and require a significant amount of reagents.Herein we introduced a novel, simple method for SnO2 synthesis using palm sugar-mediated hydrothermal for SO 2 sensing application.In this method we utilized palm sugar, an abundant natural product which is currently under-researched in material sensor synthesis, as a morphological control agent.The synthesis method's novelty and SnO2's inherent properties converge to offer an exciting potential for enhancing SO2 gas sensing capabilities.

Experiments 2.1. Tin Oxide Synthesis
SnO2 were synthesized by simple novel palm sugar-hydrothermal mediated method.Typically, 0.5 mmol SnCl2 was dissolved into 50 mL 2-propanol.Subsequently, a palm sugar was added with a SnCl2 to palm sugar molar ratio of 3:1 and then stirred for 30 minute to obtain a homogenous solution.The solution then was transferred into stainless steel autoclave linded with Teflon and heated at 160 o C for 12 hours in an oven.After natural cooling to ambient temperature, the precipate of the solution was collected by centrifugation at 4500 rpm for 7 minutes, rinsed using ethanol multiple times and dried using vacuum drier for 3 hours.The precipitate then annealed at 400•C to obtain a fine SnO2 powder.

Characterization
Crystallography information of the sample was obtained from Rigaku RINT 2500X diffractometer using monochromatic Cu-Kα radiation (λ = 1.5418Å).Field emission scanning electron microscope (FESEM) (Hitachi SU-3500) was employed to investigated morphology and microstructure information of the sample.

Sensor Fabrication and Gas Sensing Tests
The sensor used for the gas sensing was fabricate using alumina as a subtrate and silver paste (Ag) placed over the alumina substrate as electrode material.The certain amount of prepeared SnO2 was dispersed in ethanol to form slurry. Subsequently the slurry was adhered to the electrode to form a thick film and dried at ambient temperature.The sensing performance was measured at temperature range of 250ºC to 350ºC using Omron G3PX-220EH as temperature controller.Change of sensor resistance, then was measured using Picotest M3500A digital multimeter and recorded on a computer.

SEM Analysis
Figure 1 shows SEM images of as-prepared (a) and after calcination (b) SnO2.As shown in the figure, it is confirmed that SnO2 exhibits a spherical grain morphology.Morphological analysis reveals that the as-prepared SnO2 (Fig 1a ) shows a more homogeneous and uniform structure compared to the calcined SnO2 (Fig 1b).The calcination of SnO2 at 400°C leads to particle agglomeration due to the fusion of grain boundaries as the temperature increases [22], [23].The formation of this spherical morphology is expected to be controlled by the presence of palm sugar, as it can act as a morphology controlling agent of metal oxide based sensor material [24].This result provides novel insights into the use of palm sugar in SnO2 synthesis, which has not been previously reported.

XRD Analysis
The diffraction pattern of synthesized SnO2 were analyzed to study the information regarding the phase composition.The XRD pattern of the as-prepared and after 400°C calcination of spherical SnO2 is shown in Figure 2. As shown in Figure 2, There were seven diffraction peak identified as ( 112), ( 113), ( 022), ( 115), ( 130), (311), and (314).All of the diffraction peaks are matched well with Joint Committee for Powder Diffraction Standards (JCPDS) file number 78-1063 with a rutile tetragonal phase structure [22].No impurity peaks were observed, suggesting that the sample are pure.

Figure 2. XRD Pattern of SnO2 as-prepared and after calcination
Based on the diffraction pattern of calcined SnO 2 , its shows that no new peak shift detected.however, the peak intensities are lower compared to those of the as-prepared SnO2.This could be attributed to agglomeration, resulting in an increase in the average particle size [25], [26], which in good agreement with the SEM results.

Sensing Properties
To investigate the sensing performance of calcined SnO2, corresponding changes were evaluated by exposing 50 ppm SO2 gas at working temperatures of 200, 250, and 350°C (Figure 3a).According Figure 3a, it can be observed that the response increases with the rise in temperature.The rises of response can be attributed to the increaasement of the activated surface area resulting from the elevated temperature [27].To further evaluation of sensing performance, response changes against concentrations of SO2 ranging from 10 to 100 ppm were conducted at an operating temperature of 350°C (Figure 3b).Accoding in Figure 3b, it can be observed that the sensing response increases rapidly from 6.96 to 17.15 for gas concentrations between 10 and 20 ppm, and from 26.12 to 47.52 for concentrations between 80 and 100 ppm.The linear increment occurred only between 17.15 and 26.12 ppm for gas concentrations between 20 ppm and 80 ppm.The result of response againts temperature measurement (Figure 2b) relatively produce higher response values than the other work on the same gas type, gas concentration, and environmental conditions.Liewhiran et al. [28].synthesized flame-spray-made SnO2 nanoparticles as a sensing material.The measured response to 50 ppm SO2 at temperatures of 200°C, 250°C, and 350°C were 1.26, 1.27, and 5, respectively.These result indicate that our work exhibits promising performance.

Conclusion
In this study, a novel palm sugar-mediated hydrothermal method was successfully employed to synthesize SnO2 particles for enhanced SO2 gas sensing applications.The SEM analysis shows SnO2 has spherical grain structure.XRD analysis confirmed the tetragonal phase structure of SnO2.The gas sensing experiments revealed that the synthesized SnO2 exhibited a strong response to SO2 gas, with higher temperatures enhancing the response.The presented method's simplicity, cost-effectiveness, and promising gas sensing performance make this work worthy of further consideration for in-depth study and exploration.