Sol-gel vs. green synthesis of tin oxide (SnO₂) nanoparticles: influence on morphology and gas sensing behavior

Hydrogen sulfide (H₂S) is a highly corrosive and toxic gas. Accurate detection of it at low concentrations is crucial for industrial, environmental and occupational safety. Tin dioxide (SnO₂) is one of the most commonly used n-type metal semiconductors in resistive gas sensors due to its ability to absorb surface oxygen, its thermal stability, and its high sensitivity to reducing gases such as H₂S. Two methods – the traditional method, the chemical sol-gel method, and another method aided by green tea extract (Camellia sinensis) – were used to synthesize SnO₂ semiconductor nanoparticles, in order to study the effect of the manufacturing method on the structural and optical properties and the sensing behavior of hydrogen sulfide (H₂S) gas. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDX), ultraviolet-visible absorption, and photoluminescence (PL) spectroscopy were used to characterize the nanoparticles. The gas sensing response to a concentration of 50 parts per million of hydrogen sulfide (H₂S) was evaluated over a wide range of operating temperatures. X-ray diffraction (XRD) can confirm the formation of tetragonal rutile tin oxide (SnO₂) in both samples, with average crystal sizes of 6.14 nm (green method sample) and 12.47 nm (chemical method sample). Field-emission scanning electron microscopy (FESEM) can detect a granular structure in both cases, while energy-dispersive X-ray spectroscopy (EDX) analysis revealed residues of carbon, sodium, and chlorine in the sample prepared by the green method, which are attributed to green tea extract, sodium hydroxide, and tin chloride compound (SnCl₂). The environmentally friendly tin oxide (SnO₂) produced showed an estimated optical band gap of 4.4 eV, stronger photoemission at ≈ 410 nm, and a higher gas response (Rₐ/R_g = 54 at 250 °C) compared to the chemically prepared sample (Eg = 4.8 eV, response = 47). The novelty of this work lies in establishing a clear structure that links performance and properties, where factors such as crystal size, which depends on the synthesis method, the residual surface chemical composition, and the density of oxygen vacancies hidden by similar external structures combine in the chemical response resistant to hydrogen sulfide. The non-intuitive narrowing of the optical gap in smaller green-manufactured particles can be explained by the high concentration of subband gap states associated with oxygen vacancies and defects, as evidenced by the more intense PL emission, which dominates the remaining quantum confinement effects in this volumetric range.