How to synthesize oxygen vacancies?
a. Reduction treatment
Reduction gas reduction
Treatment of metal oxides with CO, NH3, H2, etc. at high temperatures or pressures is a common strategy due to the strong reducing properties of the reducing gas causing defects in the metal oxides. By adjusting the temperature, pressure, and gas composition, OVS can be efficiently produced with varying degrees and concentrations.
Reactive Metal Reducing Agents
Inorganic NaBH4, CaH2 Organic imidazole and L-ascorbic acid are used. For example, the strongly reducing metals Li, Mg, Al and Zn are used to capture lattice oxygen in oxides to create defects. A mixture of metal oxides and dimethyl carbonate (DMC) was composed of Li powder. The mixture was ground for 1 hour and by adjusting the content of Li powder (0-5 wt%) with different concentrations of defects characterized by a progressively darker color, in the process, Li removes the oxygen from the metal oxides, forming this scheme for the generation of defective oxides at room temperature.
Electrochemical reduction process
In the electrochemical reduction process, metal ions accept external electrons to form low valence metal ions. To compensate for the charge balance, oxygen vacancies are created and static cations (C +) in the electrolyte are embedded in the metal oxide. The mechanism can be summarized as
b. High-energy particle bombardment
High-energy particles of plasma and high-energy protons can strongly interact with atoms on the surface of metal oxides, leading to the surface structural damage and oxygen vacancies reported by researchers as anoxic vacancies in metal oxides. The applied high-energy plasmonic ions were implanted to modify the surface of TiO2 nanotubes to induce specific defects.
c. Vacuum calcination treatment
Calcination vacuum activation was achieved to obtain oxygen vacancy defective structures by placing the defect free metal oxides into a vacuum furnace at temperatures above 100°C. This oxygen deficient atmosphere produces oxygen vacancies and low valent metal ions.
d. Ultrasonic treatment
Ultrasonic waves with high power density are used to disorder the surface of the metal oxides and change the electronic structure to produce oxygen vacancies.
e. Sol-gel hydroxylation
A one-pot gel combustion synthesis strategy was proposed by Sanjay et al. (J. Mater. Chem. A, 2016, 4, 5854-5858). 0.4 M titanium butanolide was mixed with 50 mL of diethylene glycol (DEG) to form a yellow glycosidic acid titanium complex gel. 14.4 mL of water was added and stirred for 15 min then the hydrated titanium glycolate gel was maintained at 300 °C for 2 h. Excess water was added to ensure sufficient hydroxylation of the titanium dioxide, and the excess hydroxylation led directly to the formation of black titanium dioxide.
f. Arc melting treatment
The oxide will be first powdered into pellets and then placed in an electric arc furnace having a closed chamber filled with Ar. (Adv. Mater. 2015, 27, 2589-2594) The high temperature arc heats the metal oxide particles to the melting point for a few seconds and then rapidly cools them to room temperature. This rapid melting and cooling process effectively immobilizes the highly concentrated defects in the oxide.
g. Summary of Synthesis Methods
According to the current literature, hydrogen reduction treatment has been extensively studied as a widespread method for synthesizing oxygen vacancies. A range of metal oxides such as titanium dioxide, zirconium dioxide, manganese dioxide, molybdenum trioxide, tungsten trioxide, etc. can be produced with high quality oxygen deficient structures using this method. However, the method can be operated at high temperatures or high pressures, which is both inconvenient and time-consuming. Chemical reductant treatment as active solution reduction and room temperature process is also a common and effective method, but it is limited by the relatively low content of oxygen vacancies. Oxygen-rich proto-vacancy metal oxides can be obtained by reactive metal reduction methods, but the operation process is usually complicated.
Energetic particle bombardment, calcination-vacuum activation, and ultrasonic treatment, which have limited applications, are the only methods by which a small fraction of metal oxides with oxygen vacancies can be synthesized. In addition, the preparation conditions have a significant impact on the formation of oxygen vacancies, as discussed and summarized below. In the hydrogen reduction method, the content of oxygen vacancies in the metal oxides increases with increasing temperature and hydrogen pressure, while the color changes significantly from light to dark. As the amount of reducing agent increases, the concentration of oxygen vacancies increases accordingly. However, an excessive amount of reducing agent may lead to the appearance of new phases or even metallic singlet phases. In addition, the reaction time for making defective metal oxides is positively correlated with the amount of defects.
5. Oxygen Vacancy Characterization Methods
Theoretical models are constructed at the atomic and molecular level to analyze changes in electronic structure and to evaluate the catalytic mechanism of the carrier material
A number of challenges in the preparation of oxygen vacancy metal oxides still require further research:(i) Exploring simple and convenient methods for large-scale preparation of oxygen vacancy metallides requires continuous research as the currently reported methods are often time-consuming or performed under extreme conditions. (ii) Some metal oxides, such as ZrO2, Al2O3, Nb2O5, are difficult to reduce, and the concentration of oxygen vacancies in these metal oxides was relatively low in the study. Therefore, it is still challenging to find effective methods to implant a large number of oxygen vacancies efficiently. Oxygen-containing metal oxides fabricated by certain methods are poorly stabilized. When exposed to air conditions, metal oxides with oxygen vacancies are partially oxidized, which in turn reduces the concentration of oxygen vacancies. The problem of preventing the oxidation of defects also needs to be addressed.
