Synthesis of SnO2 semiconductor nanostructures doped with alkaline earth elements: Structural and optical study for photo-catalytic application

SAOULA, Imene (2024) Synthesis of SnO2 semiconductor nanostructures doped with alkaline earth elements: Structural and optical study for photo-catalytic application. Doctoral thesis, Université Mohamed Khider (Biskra - Algérie).

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Abstract

In this work, an investigation of different physical properties was carried out on pure and Ca, Sr, and Ba-doped SnO2 thin films. The investigation employed an experimental approach utilizing a low-cost method, namely the pneumatic spray pyrolysis technique. Additionally, a theoretical study was undertaken using density functional theory, finally a photocatalytic application was done. The experimental study was done by preparing a starting solution with a concentration of 0.5 mol/l using SnCl2, 2 H2O as a precursor to prepare pure SnO2 films, then CaCl2, 2 H2O; SrCl2, 6H2O, and BaCl2, 2 H2O were added to prepare Ca, Sr, and Ba-doped SnO2, respectively. All the samples were deposited into glass substrates heated to 450 °C, and characterized using the following four apparatuses: XRD, SEM & EDS (EDX), UV-vis spectrophotometer, and four-point probe. XRD analysis illustrated that all the films had a tetragonal rutile structure and a polycrystalline nature with a preferred growth orientation along <110> direction, SEM images showed an emergence of different chapes at the surface of the sample due to the incorporation of dopant elements into SnO2 lattice, which was confirmed by EDS (EDX) composition analysis. The optical transmittance increased for Ca-SnO2 and Sr-SnO2, whereas it decreased for Ba-SnO2 compared to pure SnO2 films, the band gap generally decreases just for Ca-SnO2. The electrical conductivity was increased with increasing of dopant concentrations just for Ca-SnO2, while for Sr, and Ba-doped SnO2, it was increased at medium concentrations then it was decreased again. The theoretical study was done using the Cambridge Serial Total Energy Package (CASTEP) code. The study included a comparison between high (8.33 at%) and low (4.16 at%) atomic ratio concentrations of dopants. From the structural study, it was found that SnO2 cell was expanded by adding M dopants (M = Ca, Sr, or Ba). After the comparison of band structures, it was showed that the band gap values decreased by adding dopant elements compared to pure SnO2. For the optical properties, the absorption coefficient was calculated and showed that its intensity increased by adding dopant elements to pure SnO2, and it is always high in the UV range. In the last part a photodegradation of the methylene blue dye was done using pure and Ca, Sr, and Ba-doped SnO2 thin films but just for low (2 wt%) and high (8 wt%) dopant concentration, to find that the degradation rate (efficiency) enhanced just when 8 wt% ca-doped SnO2 and 2 wt% Sr-doped SnO2 samples used while when the using of the other samples the efficiency deteriorated.

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