رسائل ماجيستير
English
2022
Studies on doped and annealed tungsten trioxide (WO3) particles
bderrezak Belabbes , Hamood Salim Said Al-Shidhani, Sumesh Pillai
كلية العلوم-جامعة السلطان قابوس · عمان
Nanostructured metal oxides such as ZnO, TiO₂, SnO₂, and WO₃ are essential in sensor applications due to their electrochemical properties and large chemically active surface areas. Among them, tungsten (VI) trioxide (WO₃) is particularly promising as a semiconductor, owing to its ability to exist in multiple crystal structures (e.g., cubic, orthorhombic, monoclinic) and its tunable bandgap (~2.5–3.6 eV). However, WO₃ exhibits a low emission quantum yield at room temperature due to non-radiative recombination processes.
It has been reported that increasing the active surface area of WO₃ enhances its application potential. Another interesting feature of WO₃ is the presence of oxygen vacancy-related photoluminescence centers. The electrochromic (EC) phenomenon refers to the reversible change in optical properties (light absorption and emission) when balanced charges are injected into or extracted from a material. WO₃ is one of the most widely studied electrochromic materials and is commonly used as an EC layer in smart windows.
The EC behavior of WO₃ depends significantly on the type and concentration of impurities present in its lattice, as well as the concentration of oxygen vacancies. Stoichiometric WO₃ contains W⁶⁺ ions in its lattice. However, the presence of electron-donating impurities or oxygen vacancies can introduce excess electrons, which localize on the 5d orbitals of W⁶⁺ cations. This results in the formation of W⁵⁺ (in doped samples) or both W⁵⁺ and W⁴⁺ ionic states (in sub-stoichiometric samples).
Earlier models attributed WO₃'s optical properties to photoinduced charge hopping from W⁵⁺ to neighboring W⁶⁺ sites. Later theories proposed polaron hopping as the mechanism due to lattice distortion. However, these models could not fully explain the impact of oxygen vacancies on the EC properties of WO₃. Recent research emphasizes that oxygen vacancies play a crucial role in determining EC behavior. The crystallographic direction of the vacancy is also important, particularly in the monoclinic phase, where bond lengths and angles vary across directions.
Oxygen vacancies can exist in three different charge states, and their optical properties can be explained by transitions between these states. While many theoretical and experimental studies have focused on emissions from the monoclinic phase of WO₃, the orthorhombic phase remains less explored.
This project aims to synthesize and characterize both pure and doped orthorhombic WO₃ using techniques such as XRD, XPS, TEM, SEM, PL, and UV-Vis spectroscopy. High-temperature annealing induces a structural phase transition from orthorhombic to monoclinic WO₃. Emission centers in the monoclinic structure are identified through theoretical predictions of charge transition levels (CTLs), and these predictions are supported by experimental data. These CTLs can also serve as references for theoretical calculations in the orthorhombic phase.
Aluminum (Al) doping modifies the bandgap and decreases emission intensity in WO₃ due to the suppression of certain transitions observed in undoped samples. This is attributed to an increase in non-radiative recombination pathways in the doped samples.
