Investigation of microstructural, optical, and photocatalytic properties of sol–gel synthesized pristine SnO2 nanoscale particles

In this research work, tin oxide nanoparticles (SnO₂ NPs) was developed to assess its photocatalytic performance against degradation of Congo red (CR) azodye under UVA-light illumination. To achieve this objective, SnO₂ NPs was designed and synthesized via a sol–gel method using stannous chloride and oxalic acid dihydrate as precursors. The un-calcined sample heated at 80 °C for 4 h (labled as SnO₂-80) and calcined catalysts at 450 °C and 650 °C for 4 h (identified as SnO₂-450 and 650 °C, respectively) were subsequently characterized by various description techniques such as SEM, TGA-MS, XRD and UV-vi-DRS for their physicochemical properties. Here, numerous methods have been explored to estimate the crystallite size and strain in the SnO₂-450 using X-ray peak profile analysis. XRD findings disclosed the formation of crystalized tetragonal-type SnO₂ phase with P4₂/mnm space groupe symmetry. All methods provide crystallite sizes within 20–30 nm for SnO-450 NPs, excluding for LSL model (69.30 nm) which proved to be invalid crystal. Therefore, H-W model is efficient and most accurate for examining microstructural characteristics, since it gave the highest value of R² (0.9031) and a decreased intrinsic strain (2.2 × 10^–3). Rietveld refinement, performed by HighScore plus software, on collected XRD patterns of SnO₂-450 was robust and convergence was achieved, yielding to low Rp (9.39%), Rwp (12.19.00%) difference indices. The goodness of fit parameter χ² was found to be lower χ² (1.52%). The crystallite size, D_XRD = 15.82818(8) nm and strain ε = 55.28 × 10^–4 were obtained. The band gap energy with a band gap of 3.35, 3.35 and 3.49 eV were obtained for the direc allowed electronic transitions at 80, 450 and 650 °C, respectively. An important blue shift in band gap from 3.35 eV (SnO₂-450) to 3.49 eV (SnO₂-650) with increasing the temperature from 450 to 650 °C was accredited to a strong quantum confinement. All the calcined samples exhibited a foamed nanostructural morphology with particles size of 400 nm and 750 nm-3 µm at calcination temperature of 450 and 650 °C, respectively. The impact of the calcination temperature on the particle size, band gap energy and adsorption efficiency showed temperature-dependent behavior while the photocatalytic process is practically not altered by operating temperature. The optimum efficiencies of 76.44, 62.45, 95.02 and 93.34% were achieved within 100 min under UVA-light using prepared SnO₂-450 and CeO₂-500 NPs and pristine ZnO and TiO₂ photocatalysts.Congo red degradation behaviors over different operating condition were in good agreement with the Langmuir–Hinshelwood kinetic model for pseudo first order reaction with optimal R² (R² = 0.0.881–0.97). Subsequently, the exceptional photocatalytic ability and versatile applications of SnO₂-450-based photocatalyst, outperforming all other degradation processes, could be can be synergysticaly explained by the the heterogeneous photocatalysis mechanism through ROS (^•OH and O₂^•−), Sn⁴⁺/Sn²⁺ redox system together with copious oxygen vacancies and large intrinsic crystal defects (Sn⁴⁺-O defects sites), as primary oxidizing agents and driving forces ultimately implicated in redox processes, by promoting UVA-light harvesting, facilitating charge separation of carriers, reducing recombination rate and thus boosting photocayatlytic effectiveness of SnO₂-450 NPs  photocatalyst.

Graphical abstract