I do not very understand why metal doped on tio2 can produce oxygen vacancies, the reason is due to charge neutrality. but what is charge neutrality and why oxygen can be moved to produce vacancies?
Dear Fay Yu,
You may want to look over info presented below:
Oxygen Vacancy
Generally, oxygen vacancy defect is a loss of oxygen atom from their respective position in the crystal lattice and mainly exist both in the bulk and on the surface or subsurface of the nanomaterials.
Resistive switching of oxides in microscopic mechanisms
The oxygen vacancies are the most common defects in oxide materials. The properties of oxides, especially electrical resistance, are affected appreciably by the concentration and distribution of oxygen vacancies [25, 26]. The RS phenomenon is also induced by a crucial process known as redox processes, which are also due to the oxygen vacancies [27]. Hence, the important thing here is to understand how oxygen vacancies affect the resistance of a material. Basically, oxygen vacancies induce the RS phenomenon by the following mechanisms:
(1)
Generation of filament shapes by clustering the oxygen vacancies with applying an electric field.
(2)
Schottky barrier characteristics can be controlled by oxygen vacancies.
(3)
The RS phenomenon can occur inside the Schottky barrier region due to the trapping of electrons, which form the oxygen vacancies.
In MIM structure, when an electric field is applied to the electrodes, active metal ions can migrate to the inert metal through the electrolyte and then aggregate near the inert electrode. By studying scanning electron microscopic and transmission electron microscopic images, the migration and aggregation of active metal ions can be directly observed.
https://www.sciencedirect.com/topics/materials-science/oxygen-vacancy
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This work outlines an experimental and theoretical investigation of the effect of molybdenum (Mo) doping on the oxygen vacancy formation and photocatalytic activity of TiO 2 . Analytical techniques such as x-ray diffraction (XRD), Raman, x-ray photoelectron spectroscopy (XPS) and photoluminescence (PL) were used to probe the anatase to rutile transition (ART), surface features and optical characteristics of Mo doped TiO 2 (Mo–TiO 2 ). XRD results showed that the ART was effectively impeded by 2 mol% Mo doping up to 750 °C, producing 67% anatase and 33% rutile. Moreover, the crystal growth of TiO 2 was affected by Mo doping via its interaction with oxygen vacancies and the Ti–O bond. The formation of Ti–O–Mo and Mo–Ti–O bonds were confirmed by XPS results. Phonon confinement, lattice strain and non-stoichiometric defects were validated through the Raman analysis. DFT results showed that, after substitutional doping of Mo at a Ti site in anatase, the Mo oxidation state is Mo ⁶⁺ and empty Mo- s states emerge at the titania conduction band minimum. The empty Mo- d states overlap the anatase conduction band in the DOS plot. A large energy cost, comparable to that computed for pristine anatase, is required to reduce Mo–TiO 2 through oxygen vacancy formation. Mo ⁵⁺ and Ti ³⁺ are present after the oxygen vacancy formation and occupied states due to these reduced cations emerge in the energy gap of the titania host. PL studies revealed that the electron–hole recombination process in Mo–TiO 2 was exceptionally lower than that of TiO 2 anatase and rutile. This was ascribed to introduction of 5 s gap states below the CB of TiO 2 by the Mo dopant. Moreover, the photo-generated charge carriers could easily be trapped and localized on the TiO 2 surface by Mo ⁶⁺ and Mo ⁵⁺ ions to improve the photocatalytic activity.
https://www.researchgate.net/publication/339159675_Mo_doped_TiO2_impact_on_oxygen_vacancies_anatase_phase_stability_and_photocatalytic_activity
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The TiO2 nano-based composite photocatalyst is best known for application in solving the recent issues related to energy and environmental purification. Due to the low cost, nontoxicity, chemical stability and high efficiency of TiO2, it is unquestionably one of the most considered materials in environmental treatment. In this systematic review, we reveal the outstanding potential of oxygen vacancy in photocatalysis, and discuss the contemporary advancement in the photocatalytic activities, productivity, preparation methods and oxygen vacancy of the TiO2 nano-based composite photocatalyst for environmental treatment and energy as well as wastewater treatment. This exposé is anticipated to enlighten researchers and engineers on the specific management and assessment of the environment, which warrants prospective research into developing appropriate mechanisms for energy, wastewater treatment and environmental purification.
https://pubs.rsc.org/en/content/articlehtml/2018/ra/c8ra05117h
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